sql: missing castfunc in pg_proc

[knz]

Found while working on #88061.

Describe the problem

The castfunc column contains all NULLs; instead it should contain the OID of the built-in functions that do the conversion.

To Reproduce

SELECT pg_catalog.format_type(castsource, NULL) as src,
 pg_catalog.format_type(casttarget, NULL) as dst,
 proname AS function
FROM pg_cast c
LEFT JOIN pg_proc p ON p.oid = c.castfunc

Expected behavior

The output for e.g. the conversion from boolean to text should be proname = 'text'.

cc @rafiss for triage.

Jira issue: CRDB-23242

Epic CRDB-17785

[rafiss]

We currently do not use built-in functions to perform casts, though @jordanlewis just started to explore that idea in #94859

[rafiss]

closed by #97093 - thanks @otan !