[7.x] Add mergeFillable and mergeGuarded

[hailwood]

These new methods are to be inline with the mergeCasts method that was merged a few months ago.

They're mostly useful in traits that add new attributes for example

trait HasActiveVisit
{
    protected function initializeHasActiveVisit()
    {
        $this->mergeFillable([
            'is_indefinite',
            'finished_at'
        ]);

        $this->mergeCasts([
            'is_indefinite' => 'bool',
            'finished_at'   => 'datetime'
        ]);
    }

    public function scopeActive(Builder $query)
    {
        $query->where('is_indefinite', false)->whereNull('finished_at');
    }
}