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

[andrewsu]

To incorporate the PharmGKB API as described in biothings/biothings_explorer#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: biolink/biolink-model#1236

[andrewsu]

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