Implement fetching of existing cases

[holtgrewe]

Is your feature request related to a problem? Please describe.
In the case that submissions must be updated, we have no way of fetching the data.

Describe the solution you'd like
Add support for resolving SCV to RCV/VCV. The RCV/VCV can then be fetched as XML and parsed for the SCV data. It will be beneficial to have an NCBI key for the efetch.

Describe alternatives you've considered
N/A

Additional context

• https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=clinvar&term=SCV002495727
• https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=clinvar&rettype=vcv&id=VCV000060725

[holtgrewe]

The VCV format can now be parsed after #201