Use transcript info from mehari server

[holtgrewe]

Is your feature request related to a problem? Please describe.
Depending on dotty for transcript exon information adds a dependency from varfish to dotty which is not necessary.

Describe the solution you'd like
After implementing varfish-org/mehari#339, we should switch over from dotty exons to mehari exons.

Describe alternatives you've considered
N/A

Additional context

• Add server endpoint for serving a gene's transcripts varfish-org/mehari#339