add PharmGKB mappings for genes, diseases, variants, and chemicals

[andrewsu]

To incorporate the PharmGKB API as described in https://github.com/biothings/BioThings_Explorer_TRAPI/issues/556, we need node normalizer to resolve PharmGKB identifiers for genes, diseases, variants, and chemicals. Examples of each:

Gene: BRAF - https://www.pharmgkb.org/gene/PA25408 Disease: leukemia - https://www.pharmgkb.org/disease/PA444750 Variant: rs113488022 - https://www.pharmgkb.org/variant/PA166157522 Chemical: imatinib - https://www.pharmgkb.org/chemical/PA10804

Mappings can be found in the "Primary Data" section of https://www.pharmgkb.org/downloads

Related issue to add prefixes to biolink model: https://github.com/biolink/biolink-model/issues/1236

[andrewsu]

Note that the prefixes have now been added to Biolink model in https://github.com/biolink/biolink-model/issues/1236, which was probably a prerequisite for this issue.