Introduce protobuf schemas for seqvar query output

[holtgrewe]

Is your feature request related to a problem? Please describe.
At the moment, we serialize SequenceVariant using serde to JSON in the CSV output of seqvar query. It would be good to use protobuf here so we can reuse the schema definition.

Describe the solution you'd like
Create a protobuf schema for SequenceVariant and write it out using pbjson where we currently serialize to JSON with serde.

Describe alternatives you've considered
N/A

Additional context

• Dependencies:
  • Refactor seqvar query query schema and use protobuf for input #373
  • Refactor internal data structures of seqvar query schema #374