Instantiate DRSMetaresolver from external data rather than hard-coded in constructor

[ianfore]

Options include

• Local file
• GA4GH registry
  See the following for DRS entries in registry http://registry.ga4gh.org/v1/services?type=org.ga4gh:drs:*
• Possibly identifiers.org or Bioregistry.io (needs investigation)

Note that client sub-class required depends on the server.
There may be sufficient information in registry to drive this.

[briandoconnor]

Need a ticket on DRS repo for seeing what's happening with n2t.org and bioregistry.io. For now Michael L will just cache info from identifiers.org

[ianfore]

Merged in @MichaelLukowski 's standalone resolver class in resolver.py

Will look at incorporating the capability into DRSMetaresolver.

Note that we still have the problem that the method of authorization is host specific and identifiers.org does not give us any information about that. This is the sub-class issue mentioned above. Besides the service information from GA4GH registry the /service-info endpoint on the server itself may be useful.