Where and how to define the `data_proxy` interface

[jsstevenson]

https://github.com/ga4gh/vrs-python/blob/main/src/ga4gh/vrs/dataproxy.py (always a tell when the code references "VR" and not "VRS")

Most of this is copy-pasted from SeqRepo. This seems less than ideal, but I think we probably didn't want to depend on SeqRepo for the dataproxy if it could be avoided.

• Is that still necessary? Can't we install SeqRepo and just use some of the API library, not the actual data?
• ~What about refget? It looks... a lot like the dataproxy API, and it's a GA4GH thing. Can we just define a refget API? Seqrepo-rest-server already provides refget output IIRC~ from what I hear refget is missing something that we expect from the dataproxy regarding alias lookup
• Can we just define it once in a central location and have both SeqRepo and VRS Python import it?

[jsstevenson]

Didn't think much of it at the time, but the new derive_refget_accession method that was added to the VRS DataProxy class does mean you have to be careful about how you're constructing it in cases where you need a singleton instance shared across multiple resources (e.g. Cool Seq Tool, Variation Normalizer, etc)