Add server endpoint for serving a gene's transcripts

[holtgrewe]

Is your feature request related to a problem? Please describe.
In particular for supporting reev-frontend-lib in VarFish, we need to serve transcripts (exon list). This removes the requirement to host the additional dotty service.

Describe the solution you'd like
Add an endpoint that serves all transcripts for a given HGNC ID.

Describe alternatives you've considered
N/A

Additional context

• Use transcript info from mehari server bihealth/reev-frontend-lib#115
• Ideally, use protocol buffer generated classes with the JSON serialization only for the response so we can easily augment the frontend in reev-frontend-lib.