Add parser/writer for GAF

[cmungall]

Many groups use GAFs for phenotypes as well as GO - e.g. planteome (cc @jhpoelen)

Conversely, pxf is generic enough for GO annotations (which are essentially normal, evolved phenotypes for genes)

It would be useful to read/write from GAFs into the phenopacket datamodel.

There are also a family of similar formats which could follow the same pattern. E.g. PAFs:
phenopackets/phenopacket-format#67

And of course the HPOA (non-phenote) TSVs