Run set-returning functions in the target list

[tamnd]

Part of #3.

This change lets a query call a set-returning function in its target list, as the ProjectSet node of PostgreSQL does. The view pg_stats_ext_exprs and the body of _pg_expandarray in information_schema need this, and client tools often send such queries.

What changes

• The analyzer marks each call of a function that returns a set. check_srf_placement gives the error of PostgreSQL for a call in WHERE, HAVING, FILTER, LIMIT, OFFSET, VALUES, a JOIN condition, a default, a check, an index and the parameters of EXECUTE.
• A call inside CASE or COALESCE, and a call inside the arguments of an aggregate, gives the error of PostgreSQL with its hint and position.
• The executor puts the calls in levels, as split_pathtarget_at_srfs does. A call is one level above the highest call in its arguments.
• The calls of one level run side by side. A call with fewer rows gives nulls. When all calls of a level give no rows, the input row gives no rows.
• The calls run after the aggregates and HAVING, and before ORDER BY, DISTINCT and LIMIT.
• A function with more than one OUT parameter gives a row value.
• The planner does not pull up a subquery with such a call, and it does not change EXISTS of such a subquery, as PostgreSQL does.
• The view pg_stats_ext_exprs is now in the catalog with the OID of PostgreSQL 19, so all the views of system_views.sql are there. It needs row_security_active and pg_get_statisticsobjdef_expressions, which this change adds with pg_get_statisticsobjdef and pg_get_statisticsobjdef_columns. rupg has no objects of extended statistics, so these give null.
• pg_get_viewdef does not put parentheses around a field before a subscript, as ruleutils.c does.
• A call in GROUP BY and a call that gives a record with a polymorphic OUT type give the error 0A000 for now.

Test

• The new session test target_srf runs a script and compares the result with the output of PostgreSQL 19 for the same script in a new database. It covers single calls, calls side by side, nested calls, calls inside expressions, strict calls with a null argument, aggregates, HAVING, ORDER BY, DISTINCT ON, LIMIT, subqueries, EXISTS, IN, row values and each error of a call in a wrong place.
• The session test system_views now covers pg_stats_ext_exprs with its definition, and row_security_active and the functions of extended statistics with each error. The expected output is from PostgreSQL 19.
• srvtest passed on server1: fmt, layers, style, clippy, all the workspace tests, the generated file checks and the docs.