Support cast "signed" / "unsigned" for MySQL

[Patrick-Ullrich]

Currently cast does not accept signed and unsigned.

However in mysql:

SELECT cast("0" AS integer)

will result in:
Query 1 ERROR at Line 1: : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'integer)' at line 1

The correct syntax be:

SELECT cast("0" AS signed)

or

SELECT cast("0" AS unsigned)