Make database optional in grant resource

[ClemontX]

Hey guys,

i don't have much experience with go and in developing terraform providers. I wanted to use this provider to create a simple privilige management. I've discovered the problem that if you want to grant a role to a user, you must define a database for the grant resource. This isn't needed by the latest mysql and mariadb versions (tested). A sample in the MariaDB documentation is here.

So i changed the line in the code which declares the database as required and changed it to optional.

I hope i did everything the right way.

Greets Carl