Create a means for applications using mx-bluesky parameter models to add defaults

[dperl-dls]

• I dislike that we need to have a subclass to basically just fill in the defaults. I feel like you should be able to do something like the way we fill in the devices using the BlueAPI context where we also fill in these constants from the context (which picks them out the appropriate dodal.beamline)

Originally posted by @DominicOram in DiamondLightSource/hyperion#1537 (review)