Add `AllelicState` into `org.phenopackets.phenopackettools.builder.constants` package?

[ielis]

The VariationDescriptorBuilder allows to set the allelicState field via heterozygous(), homozygous() etc. The methods contain hard-coded GENO constants. However, it may be useful to define the allelic state constants in a new org.phenopackets.phenopackettools.builder.constants.AllelicState class.

[pnrobinson]

good suggestion, done!