Implement HGVS parsing for protein sequences

[holtgrewe]

Is your feature request related to a problem? Please describe.
Parsing HGVS protein sequences has not been implemented yet. It would be useful to have this (cf #490, #491, #493).

Describe the solution you'd like
Add implementation and tests in the same way as we have for nucleic acid changes.

Describe alternatives you've considered
N/A

Additional context
N/A