Provide endpoint alternative with OpenAPI schema for /seqvars/csq

[holtgrewe]

Is your feature request related to a problem? Please describe.
Right now, /seqvars/csq does not have an OpenAPI schema.

Describe the solution you'd like
Add an alternative implementation (sharing as much as possible with the original code) that provides an OpenAPI schema. Keep the old one until all clients calling the old API are migrated.

Describe alternatives you've considered
N/A

Additional context

• Provide endpoint alternative with OpenAPI schema for /genes/txs #605
• Provide endpoint alternative with OpenAPI schema for /strucvars/csq #607