plpgsql: support DEFAULT syntax in the declaration

[yuzefovich]

This two definitions are equivalent, but we only currently support the latter:

DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p() LANGUAGE PLpgSQL AS $$
DECLARE
    var TEXT DEFAULT 'test';
BEGIN
    RAISE NOTICE '%', var;
END;
$$;

DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p() LANGUAGE PLpgSQL AS $$
DECLARE
    var TEXT = 'test';
BEGIN
    RAISE NOTICE '%', var;
END;
$$;

Jira issue: CRDB-37793