plpgsql: support accessing variables via label.var_name pattern

[yuzefovich]

This works in Postgres:

DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p() LANGUAGE PLpgSQL AS $$
BEGIN
    <<label>>
    DECLARE
      v text := 'test';
    BEGIN
        RAISE NOTICE '%', label.v;
    END;
END;
$$;

Note that support for this access pattern makes sense the most when variable shadowing is allowed (#117508).

Relatedly, it's possible to access the input arguments via routine-name.arg-name pattern:

DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p(param1 INT, param2 TEXT) LANGUAGE PLpgSQL AS $$
BEGIN
    RAISE NOTICE '% %', p.param1, p.param2;
END;
$$;
CALL p(3, 'foo');

Jira issue: CRDB-37800