add endpoint to return metadata about an annotation via model identifier

[sierra-moxon]

see: geneontology/helpdesk#450 (comment)

[kltm]

@sierra-moxon Where did this come from again?

[sierra-moxon]

per your feedback in slack - please prioritize as you see fit.

[kltm]

Spinning up with: geneontology/project-management#80
https://github.com/orgs/geneontology/projects/142