API endpoint to query languages with sentences but not enabled in CV

Currently we have a few more languages in the Sentence Collector than are enabled on Common Voice. We should create an endpoint that lists those languages so we can query it and see when there are languages being worked on and we don't know about it yet.

Create endpoint at /languages/additional
Endpoint should list (JSON) languages that are not added to https://raw.githubusercontent.com/common-voice/common-voice/main/locales/all.json but have sentences in the database attached to that language
If performance allows it, it would also be interesting to see how many sentences each of those languages have already in Sentence Collector
Add endpoint and documentation for it to the Swagger file

common-voice / sentence-collector

API endpoint to query languages with sentences but not enabled in CV #475