Breakout variable lookup into separate files and tests (#1921)

## Changes

While looking into adding variable lookups for notification destinations
([API][API]), I found the codegen approach for different classes of
variable lookups a bit complex. The template had a custom field override
(for service principals), the package had an override for the cluster
lookup, and it didn't produce tests.

The notification destinations API uses a default page size of 20 for
listing. I want to use a larger page size to limit the number of API
calls, so that would imply another customization on the template or a
manual override.

This code being rather mechanical, I used copilot to produce all
instances of the resolvers and their tests (after writing one of them
manually).

[api]: https://docs.databricks.com/api/workspace/notificationdestinations

## Tests

* Unit tests pass
* Manual confirmation that lookups of warehouses still work

.codegen.json

@@ -5,8 +5,7 @@
     },
     "batch": {
         ".codegen/cmds-workspace.go.tmpl": "cmd/workspace/cmd.go",
-        ".codegen/cmds-account.go.tmpl": "cmd/account/cmd.go",
-        ".codegen/lookup.go.tmpl": "bundle/config/variable/lookup.go"
+        ".codegen/cmds-account.go.tmpl": "cmd/account/cmd.go"
     },
     "toolchain": {
         "required": ["go"],

.gitattributes

@@ -1,4 +1,3 @@
-bundle/config/variable/lookup.go linguist-generated=true
 cmd/account/access-control/access-control.go linguist-generated=true
 cmd/account/billable-usage/billable-usage.go linguist-generated=true
 cmd/account/budgets/budgets.go linguist-generated=true