Allow custom tag table name and primary key for scoped queries

[gtmassey]

Scoped queries in the HasTags trait previously hard-coded the tags.id value for the where or whereIn clauses. This PR addresses the issue by creating new methods to get the Tag class's table name and primary key name from the defined class name in the config file, or by defaulting to Tag::class.

Tests were not updated, because existing tests cover use cases.

[freekmurze]

Thanks!