feat: process ontology bridge terms

[nayib-jose-gloria]

Reason for Change

• Add CL bridge between species-specific and taxon-neutral ontology terms single-cell-curation#1002 and Add UBERON bridge between species-specific and taxon-neutral ontology terms single-cell-curation#1003
• Want to process

Changes

• update schemas to support specifying ontology term 'cross ontology mapping' files that map cross-ontology equivalent terms
• update ontology asset builder to download these files---assume SSSOM.TSV format for now
• function to map cross-ontology terms from SSSOM and add to ontology asset files

Testing steps

• Either list QA steps or reasoning you feel QA is unnecessary
• Describe how you made sure to know that your changes worked. Should allow someone else to go verify your code without in depth knowledge.

Notes for Reviewer

• TODO: API support for getting bridge terms given an ontology term ID, where applicable
• TODO: leverage new functions in cellxgene-schema to map organism_cell_type and organism_tissue terms to CL and UBERON terms respectively for experimental organisms
• TODO: long-term--consider whether to be prescriptive about cross-ontology-mapping filetype (we only accept SSSOM), or be robust to other options (parse xref in ontology JSONs directly, accept bridge files, or something else that emerges)