Support custom pivot models

[SlyDave]

Custom Pivot models are being included as standard models

(likely because they ultimately extend Model::class)

Relations that actually use these Custom model classes aren't noted as such

The above is an example of

    public function vessels(): BelongsToMany
    {
        return $this->belongsToMany(Vessel::class, 'users_vessels')->using(UserVesselPivot::class);
    }

I think if a class is of Pivot::class it can be ignored for layout. But as the Pivot class has its own fillable etc for use with withPivot() it could also be creatable via this package when defining relations - maybe an "Add Custom Pivot Class" option then provide the same model creation interface that already exists.

[dedobbin]

I'm giving it a shot to implement this