Add OxO provider for mappings

[nichtich]

OxO2 (just to be released, see https://wwwdev.ebi.ac.uk/oxo2) calculates inferred mappings, these should be accessible too.

Example of a mapping: https://wwwdev.ebi.ac.uk/oxo2/mapping/e91b72b3-e090-3e88-b0b0-366c1c45a6c7

[nichtich]

OxO has an API and a new API version is planned so not sure which version to build on.

Some sample queries to API v2:

• https://wwwdev.ebi.ac.uk/oxo2/api/v2/ontologies - aka terminologies
• https://wwwdev.ebi.ac.uk/oxo2/api/v2/mapping-sets - aka mapping registries
• https://wwwdev.ebi.ac.uk/oxo2/api/sssom/mappings/e9210e09-6734-31e4-b1cd-a6601daac86a

Some sample queries to SSSOM API (newer):

• https://wwwdev.ebi.ac.uk/oxo2/api/sssom/mapping_sets
• https://wwwdev.ebi.ac.uk/oxo2/api/sssom/mappings/e9210e09-6734-31e4-b1cd-a6601daac86a
• https://wwwdev.ebi.ac.uk/oxo2/api/sssom/stats

Data model is based on SSSOM, so only 1-to-1 mappings are supported.

SSSOM API also has a specification, so provider could be "SSSOM Provider".

[nichtich]

Requires EBISPOT/oxo2#113 to map SSSOM Curies to actual URIs.

[nichtich]

• Currently 233 out of 1340 ontologies at https://wwwdev.ebi.ac.uk/oxo2/api/v2/ontologies have an URI, these can be used for getSchemes.
• There are 366 mapping sets at https://wwwdev.ebi.ac.uk/oxo2/api/v2/mapping-sets
• Query mappings, e.g. by source concept: https://wwwdev.ebi.ac.uk/oxo2/api/sssom/mappings/object_id/MESH:D002386
  or https://wwwdev.ebi.ac.uk/oxo2/api/sssom/mappings?filter=object_id%7Ceq%7CMESH:D002386 (filter is repeatable)
• Search mappings at https://wwwdev.ebi.ac.uk/oxo2/api/sssom/mappings/search with more complex payload, e.g. to search for mappings from a specific terminology (see e.g. https://wwwdev.ebi.ac.uk/oxo2/search/_map?from=BARTOC)