sql: SHOW FUNCTIONS FROM pg_catalog is slow

[knz]

Describe the problem

See the title.

This may be fallout from #94339? unsure

To Reproduce

cockroach demo then show functions from pg_catalog;

Expected behavior

Lists UDF overloads in pg_catalog schema.

cc @jordanlewis @postamar for triage.

Jira issue: CRDB-23115

[knz]

Oh I stand corrected, it does terminate eventually, after a full minute of waiting.
There's only 1904 rows. It shouldn't take 1 minute to generate 2k rows.

[jordanlewis]

SHOW FUNCTIONS currently runs 2 slow projections:

 select pg_get_function_result(oid) from pg_proc;
select pg_catalog.pg_get_function_identity_arguments(oid) from pg_proc;

These weren't edited by my recent PR, but we should definitely do something about this.

[jordanlewis]

Redefining these as SQL functions speeds them up enormously!