CBG-4213: add attachment migration api

[gregns1]

CBG-4213

• Addition of the api end point to trigger attachment migration and to get status of it
• Move tests into separate package in rest
• Api docs for this new endpoint

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2778/

[github-actions]

Redocly previews

• Admin API
• Public API
• Metric API
• Diagnostic API

[bbrks]

Looks good, one nitpick but feel free to merge as-is