Add back missing response models to openapi.json

[ehclark]

Split off from #85

Same underlying problem as cancervariants/variation-normalization#543

Changed the response models to return Dict instead of the types from ga4gh.vrs.models. This allows the full, dereferenced VRS object to be returned from the API endpoints instead of the version produced by the serialization logic embedded in the model hierarchy.

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[jsstevenson]

Closing in on a resolution in ga4gh/vrs-python#435