Mapping abstraction + custom plugin system

[jsstevenson]

At present, AnyVar automatically performs liftover between GRCh37 and GRCh38 and retains mappings to support cross-assembly variant retrieval. Liftover is currently performed using chain files provided by the UCSC Genome Browser. We will introduce a plugin architecture that can incorporate community-developed liftover and mapping plugins, enabling broader support for alternative reference assemblies. This approach will also facilitate registration of variants defined on model organism genomes or bespoke experimental reference sequences, with mappings back to human reference sequences where appropriate.

We'll probably want to a) define an abstraction for mapping services, and b) make them injectable at runtime somehow. Maybe even selectable upon ingest.