Schematize resource_grant

What does this PR do?

The resource_grant resource is very convoluted. For instance, the database might not be a database, and the table might not be a table (e.g. https://github.com/petoju/terraform-provider-mysql/pull/98/files#r1451607847).
To fix this, I introduced various structs, which makes the logic for parsing grants much easier to read.
I additionally added first-class support for procedure grants (https://github.com/petoju/terraform-provider-mysql/issues/99).
This sets the stage for subsequent PRs to actually create separate Terraform resources for each of these things, which would make the logic far easier to manage.