Adding support for casting to SIGNED

[tomershay]

The following wasn't supported by the parser this far, because of the "AS SIGNED" expression:
SELECT CAST(contact_id AS SIGNED) FROM contact WHERE contact_id = 20;

Similar expressions like AS SIGNED INTEGER are supported, but the parser expects a data type after the SIGNED keyword, which isn't mandatory according to the docs.

More examples can be seen in the documentation (search for "as SIGNED"):
https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html

[coveralls]

Coverage remained the same at 83.508% when pulling 51d41d7 on tomershay:support_cast_to_signed into 596e631 on JSQLParser:master.