Add genotype to phenopacket

[pnrobinson]

Consider

1. VMC
2. HGVS
3. VCF like
4. combination of abov e

[julesjacobsen]

Beacon or MME implementation:

We already have this:

https://github.com/phenopackets/phenopacket-reference-implementation/blob/master/src/main/java/org/phenopackets/api/model/entity/Variant.java