feat!: make mapping_type optional

[jsstevenson]

close #462

I think this was a mistake in how we designed the endpoints -- we want a way to retrieve all mappings, but a path param can't be optional. It also makes more sense to me that mapping_type would be like a filter on the /mappings/ path rather than a subtype.

So the new path looks like GET /objects/<vrs_id>/mappings?mapping_type=liftover_to or GET /objects/<vrs_id>/mappings

Alternatives considered

• Rather than re-engineering this endpoint, just add a new, general GET /objects/<vrs_id>/mappings endpoint that retrieves all mappings. I went ahead with just changing the existing endpoint for the reasons above.