Changed definitions of overridden functions to match the original

[AdalbertMemSQL]

In https://github.com/singlestore-labs/singlestoredb-laravel-driver/pull/46/ I overrode 4 functions of the MySqlGrammar and added a return type to the definition. Occurred that this is a breaking change as all users that extended from our class and overridden the same functions - also should add the return type.

[carlsverre]

I'll land and ship this now. But in parallel is it possible to add some tests to ensure we don't break this in the future?