Support FOR <statement> RETURNING in PSQL

[asfernandes]

This complements #6815 to allow non-singleton RETURNING in PSQL.

It should support statements:

• INSERT ... SELECT ...
• Searched UPDATE
• Searched DELETE
• MERGE
• UPDATE OR INSERT

[mrotteveel]

Inspired by my proposal in #7573, consider borrowing part of my proposed syntax there, so instead of implementing it as

FOR UPDATE ... RETURNING column INTO var DO

Consider using something like

FOR TABLE(UPDATE ... RETURNING column) INTO var DO

This may not make too much sense in isolation, but - in my opinion - results in a more consistent syntax if both this and #7573 are implemented.