plpgsql: implement set-returning PLpgSQL functions

[DrewKimball]

Postgres PLpgSQL allows for RETURN NEXT and RETURN QUERY syntax in order to return multiple rows from a PLpgSQL set-returning function. See the section 43.6.1.2. RETURN NEXT And RETURN QUERY in the postgres docs: https://www.postgresql.org/docs/current/plpgsql-control-structures.html

This issue tracks implementing that syntax. This will likely require implementing a new relational operator, RelCase, which conditionally executes relational expressions. This is necessary because PLpgSQL control flow requires the ability to conditionally execute functions, and the current scalar Case operator cannot handle set-returning functions.

Jira issue: CRDB-28933

[DrewKimball]

This work is not slated for 23.2.