Add get_sssom_mappings_by_curie to BioportalImplementation class

[pkalita-lbl]

This method uses the /ontologies/:ontology/classes/:cls/mappings endpoint to get mappings and reformats them in SSSOM format.

Caveats:

• The BioPortal API has an issue where a lot of duplicate mappings are reported. We are not trying to resolve that duplication here and instead just passing the results through.
• The BioPortal API reports a source field for each mapping and we are manually translating that to a SKOS term, but this may be up for refinement
• The method will accept a CURIE as defined by the MappingProviderInterface but since the BioPortal API generally deals with URIs and not CURIEs the SSSOM subject_id and predicate_id fields will be URIs

[cmungall]

yeh it's not super clear which skos property to use (other than SAME_URI), I think this is good enough to get started!