feat: Extract Foreign Key from Payload

[cjmellor]

This PR aims to clean up some ways that the package handles things.

1. Until now, if you created a Model that would be approved, e.g.

Comment::create(['body' => 'Hi']);

There would be a constraint violation when trying to approve the Model if it had a foreign key, i.e. a Comment model would normally have a relationship to a User model. Now, as long as the user adds the foreign key, everything should be okay, e.g.

Comment::create(['body' => 'Hi', 'user_id' => auth()->id()]);

If you were to add a comment via a relationship, e.g.

$user->comment()->create(['body' => 'Hi']);

there will be no difference, as this will always send the foreign key along with it.

2. When creating a new Model (to be approved), and it had a relationship (like a user_id) then that property would be added to the new_data JSON payload, and I just didn't like how that looked, so I have extracted it to its own column in the approvals table. This does mean the user needs to run a new migration though when using the next version (should be v1.5.0)