[6.x] Implemented parse ID on find method for many to many relation

[pauladams8]

I implemented the parseId() on the find() method for the BelongsToMany trait, meaning that a related model can be found using either a primary key, or an existing model instance.

Example: $project->users()->find($user); can be used as well as $project->users()->find($user->id);.

This is useful looking up pivot table data for two existing model instances: $project->users->find($user)->pivot->role;.

I think this is more consistent with other methods in the BelongsToMany trait, such as updateExistingPivot(), which implements parseId() meaning a model instance or ID can be passed.