Generalize locking clause (#759)

Postgres supports more generalized locking clauses, for example:
FOR UPDATE OF <table_name> SKIP LOCKED

also, multiple locking clauses. Generalize the parser to support these.

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

tests/sqlparser_postgres.rs

@@ -1512,7 +1512,7 @@ fn parse_array_subquery_expr() {
             limit: None,
             offset: None,
             fetch: None,
-            lock: None,
+            locks: vec![],
         })),
         expr_from_projection(only(&select.projection)),
     );