[12.x] Add refreshToken relation to Token model

[gdebrauwer]

In my project, when I user logs out, I need to revoke the access token and the refresh token linked to that access token. Currently I have to this in the following way:

auth()->user()->token()->revoke();
RefreshToken::where('access_token_id', auth()->user()->token()->id)->first()->revoke();

This PR makes that a bit easier by adding a 'refreshToken' relation to the access token model:

auth()->user()->token()->revoke();
auth()->user()->token()->refreshToken->revoke();