Add validation for single node clusters (#1909)

## Changes
This PR adds a warning validating that the configuration for a single
node cluster is valid for interactive, job, job-task, and pipeline
clusters.

Note: We skip the validation if a cluster policy is configured because
the policy is likely to configure `spark_conf` / `custom_tags` itself.

Note: Terrform originally only had validation for interactive, job, and
job-task clusters. This PR adding the validation for pipeline clusters
as well is new.

This PR follows the same logic as we used to have in Terraform. The
validation was removed from Terraform because we had no way to demote
the error to a warning:
databricks/terraform-provider-databricks#4222

### Background
Single-node clusters require `spark_conf` and `custom_tags` to be
correctly set in the cluster definition for them to function optimally.
The cluster will be created even if incorrectly configured, but its
performance will not be great.

For example, if both `spark_conf` and `custom_tags` are not set and
`num_workers` is 0, then only the driver process will be launched on the
cluster compute instance thus leading to sub-optimal utilization of
available compute resources and no parallelization across worker
processes when processing a spark query.

### Issue

This PR addresses some issues reported in
#1546

## Tests
Unit tests and manually.

Example output of the warning:
```
➜  bundle-playground git:(master) ✗ cli bundle validate
Warning: Single node cluster is not correctly configured
  at resources.pipelines.bar.clusters[0]
  in databricks.yml:29:11

num_workers should be 0 only for single-node clusters. To create a
valid single node cluster please ensure that the following properties
are correctly set in the cluster specification:

  spark_conf:
    spark.databricks.cluster.profile: singleNode
    spark.master: local[*]

  custom_tags:
    ResourceClass: SingleNode
  

Name: foobar
Target: default
Workspace:
  User: [email protected]
  Path: /Workspace/Users/[email protected]/.bundle/foobar/default

Found 1 warning
```

bundle/config/validate/single_node_cluster.go

@@ -0,0 +1,137 @@
+package validate
+
+import (
+	"context"
+	"strings"
+
+	"github.com/databricks/cli/bundle"
+	"github.com/databricks/cli/libs/diag"
+	"github.com/databricks/cli/libs/dyn"
+	"github.com/databricks/cli/libs/dyn/convert"
+	"github.com/databricks/cli/libs/log"
+)
+
+// Validates that any single node clusters defined in the bundle are correctly configured.
+func SingleNodeCluster() bundle.ReadOnlyMutator {
+	return &singleNodeCluster{}
+}
+
+type singleNodeCluster struct{}
+
+func (m *singleNodeCluster) Name() string {
+	return "validate:SingleNodeCluster"
+}
+
+const singleNodeWarningDetail = `num_workers should be 0 only for single-node clusters. To create a
+valid single node cluster please ensure that the following properties
+are correctly set in the cluster specification:
+
+  spark_conf:
+    spark.databricks.cluster.profile: singleNode
+    spark.master: local[*]
+
+  custom_tags:
+    ResourceClass: SingleNode
+  `
+
+const singleNodeWarningSummary = `Single node cluster is not correctly configured`
+
+func showSingleNodeClusterWarning(ctx context.Context, v dyn.Value) bool {
+	// Check if the user has explicitly set the num_workers to 0. Skip the warning
+	// if that's not the case.
+	numWorkers, ok := v.Get("num_workers").AsInt()
+	if !ok || numWorkers > 0 {
+		return false
+	}
+
+	// Convenient type that contains the common fields from compute.ClusterSpec and
+	// pipelines.PipelineCluster that we are interested in.
+	type ClusterConf struct {
+		SparkConf  map[string]string `json:"spark_conf"`
+		CustomTags map[string]string `json:"custom_tags"`
+		PolicyId   string            `json:"policy_id"`
+	}
+
+	conf := &ClusterConf{}
+	err := convert.ToTyped(conf, v)
+	if err != nil {
+		return false
+	}
+
+	// If the policy id is set, we don't want to show the warning. This is because
+	// the user might have configured `spark_conf` and `custom_tags` correctly
+	// in their cluster policy.
+	if conf.PolicyId != "" {
+		return false
+	}
+
+	profile, ok := conf.SparkConf["spark.databricks.cluster.profile"]
+	if !ok {
+		log.Debugf(ctx, "spark_conf spark.databricks.cluster.profile not found in single-node cluster spec")
+		return true
+	}
+	if profile != "singleNode" {
+		log.Debugf(ctx, "spark_conf spark.databricks.cluster.profile is not singleNode in single-node cluster spec: %s", profile)
+		return true
+	}
+
+	master, ok := conf.SparkConf["spark.master"]
+	if !ok {
+		log.Debugf(ctx, "spark_conf spark.master not found in single-node cluster spec")
+		return true
+	}
+	if !strings.HasPrefix(master, "local") {
+		log.Debugf(ctx, "spark_conf spark.master does not start with local in single-node cluster spec: %s", master)
+		return true
+	}
+
+	resourceClass, ok := conf.CustomTags["ResourceClass"]
+	if !ok {
+		log.Debugf(ctx, "custom_tag ResourceClass not found in single-node cluster spec")
+		return true
+	}
+	if resourceClass != "SingleNode" {
+		log.Debugf(ctx, "custom_tag ResourceClass is not SingleNode in single-node cluster spec: %s", resourceClass)
+		return true
+	}
+
+	return false
+}
+
+func (m *singleNodeCluster) Apply(ctx context.Context, rb bundle.ReadOnlyBundle) diag.Diagnostics {
+	diags := diag.Diagnostics{}
+
+	patterns := []dyn.Pattern{
+		// Interactive clusters
+		dyn.NewPattern(dyn.Key("resources"), dyn.Key("clusters"), dyn.AnyKey()),
+		// Job clusters
+		dyn.NewPattern(dyn.Key("resources"), dyn.Key("jobs"), dyn.AnyKey(), dyn.Key("job_clusters"), dyn.AnyIndex(), dyn.Key("new_cluster")),
+		// Job task clusters
+		dyn.NewPattern(dyn.Key("resources"), dyn.Key("jobs"), dyn.AnyKey(), dyn.Key("tasks"), dyn.AnyIndex(), dyn.Key("new_cluster")),
+		// Job for each task clusters
+		dyn.NewPattern(dyn.Key("resources"), dyn.Key("jobs"), dyn.AnyKey(), dyn.Key("tasks"), dyn.AnyIndex(), dyn.Key("for_each_task"), dyn.Key("task"), dyn.Key("new_cluster")),
+		// Pipeline clusters
+		dyn.NewPattern(dyn.Key("resources"), dyn.Key("pipelines"), dyn.AnyKey(), dyn.Key("clusters"), dyn.AnyIndex()),
+	}
+
+	for _, p := range patterns {
+		_, err := dyn.MapByPattern(rb.Config().Value(), p, func(p dyn.Path, v dyn.Value) (dyn.Value, error) {
+			warning := diag.Diagnostic{
+				Severity:  diag.Warning,
+				Summary:   singleNodeWarningSummary,
+				Detail:    singleNodeWarningDetail,
+				Locations: v.Locations(),
+				Paths:     []dyn.Path{p},
+			}
+
+			if showSingleNodeClusterWarning(ctx, v) {
+				diags = append(diags, warning)
+			}
+			return v, nil
+		})
+		if err != nil {
+			log.Debugf(ctx, "Error while applying single node cluster validation: %s", err)
+		}
+	}
+	return diags
+}