Introspect comparison operators (#84)

### What

Rather than dreaming up the comparison operators that can apply to each
type we ask the database.

### How

We introduce a new configuration field, `comparisonOperatorAliases`,
which defines what names to expose database infix comparison operators
under. This cannot be derived only from introspection. The default uses
the names of graphql-engine v2.

We filter how much of the introspection data ends up in the deployment
and schema to include only types, comparison operators, and aggregation
functions that are actually used in native queries or tables, in order
to keep the size small and the contents concise and relevant.

The operators are sourced from the catalog table `pg_operator`, because
this is supported by both Postgres and CockroachDB. We could relatively
easily extend our comparison operators to cover normal prefix
predicates, but that is future work.

Answering the question "which operators are defined for a given type" is
a somewhat nuanced affair, since we have to take into account operator
overloading and implicit casts.

There is even the insular case of an operator (`SIMILAR TO` in Postgres)
being defined as syntactic sugar over `LIKE`. Since `SIMILAR TO` is
somewhat obscure (IMO) and not really very distinguished from LIKE and
`REGEX`, I've opted not to try and have it appear automatically during
configuration/introspection. (And any user who categorically needs to
use it are free to define it themselves in the deployment)

---------

Co-authored-by: Samir Talwar <[email protected]>

crates/connectors/ndc-citus/tests/query_tests.rs

@@ -108,18 +108,6 @@ mod predicates {
         insta::assert_json_snapshot!(result);
     }
 
-    #[tokio::test]
-    async fn select_where_name_similar() {
-        let result = run_query(create_router().await, "select_where_name_similar").await;
-        insta::assert_json_snapshot!(result);
-    }
-
-    #[tokio::test]
-    async fn select_where_name_nsimilar() {
-        let result = run_query(create_router().await, "select_where_name_nsimilar").await;
-        insta::assert_json_snapshot!(result);
-    }
-
     #[tokio::test]
     async fn select_where_name_regex() {
         let result = run_query(create_router().await, "select_where_name_regex").await;

crates/connectors/ndc-citus/tests/snapshots/explain_tests__select_where_name_nilike.snap

@@ -20,7 +20,7 @@ FROM
             FROM
               "public"."Album" AS "%0_Album"
             WHERE
-              ("%0_Album"."Title" NOT ILIKE cast($1 as varchar))
+              ("%0_Album"."Title" !~~* cast($1 as varchar))
             ORDER BY
               "%0_Album"."AlbumId" ASC
             LIMIT

crates/connectors/ndc-citus/tests/snapshots/explain_tests__select_where_variable.snap

@@ -20,7 +20,7 @@ FROM
             FROM
               "public"."Album" AS "%0_Album"
             WHERE
-              ("%0_Album"."Title" LIKE $1)
+              ("%0_Album"."Title" ~~ $1)
             ORDER BY
               "%0_Album"."AlbumId" ASC
           ) AS "%2_rows"