Allow DRYPermissionsField to return permissions for related objects

[jordanmkoncz]

It would be useful if DRYPermissionsField could return permission not only for the object itself, but also related objects. Currently, you can use additional_actions to specify additional permissions to return, e.g. I have a model BookingAvailability where I'm including some additional permissions:

permissions = DRYPermissionsField(
    additional_actions=[
        'make_available',
        'make_busy',
    ]
)

It would be good if I could also do the following:

permissions = DRYPermissionsField(
    additional_actions=[
        'make_available',
        'make_busy',
        'booking.make_cancelled',
    ]
)

Where booking.make_cancelled refers to the make_cancelled permission of the related Booking (BookingAvailability has a ForeginKey relationship to a Booking).

This would be very similar to how other serializer fields (like DRF's CharField) are able to use related objects, for example:

booking_title = serializers.CharField(source='booking.title')

Where booking_title will be the title of the related Booking.