sql: support record input types in plpgsql udfs

[rharding6373]

We are currently unable to support RECORD input arguments in UDFs using the PLpgSQL language. The underlying types in the RECORD need to be determined at assignment time, but in UDFs we currently determine parameter types from what is provided at function creation time. This could probably be fixed by using the resolved type of the argument after argument resolution when building the UDF. In addition, we have some issues with casting to type RECORD that may prevent the usefulness of this feature.

An example query:

CREATE FUNCTION f_105120(p1 RECORD) 
  RETURNS RECORD 
  STABLE 
  LANGUAGE PLPGSQL AS $$ 
    BEGIN
       RETURN p1; 
    END $$;

SELECT
  col
FROM
  (VALUES (f_105120(('hello':::STRING,)::RECORD)), (())) AS t (col)
ORDER BY col;

Jira issue: CRDB-29157
Epic: CRDB-34165

[rharding6373]

This particular example may be blocked by #105669

[mgartner]

Let's determine if this is an acceptable limitation for v23.2.

[michae2]

We think this is an acceptable limitation for v23.2.

[blathers-crl]

Hi @yuzefovich, please add branch-* labels to identify which branch(es) this GA-blocker affects.

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