Replace Athena with RxNav in FHIR mapping YAML files

[karafecho]

This issue is to suggest that we redirect our medication mapping effort from Athena to RxNorm. I propose the following plan:

1. Test a small number of medication variables to see if we can automate the process via the RxNav API endpoint.
2. If successful, redo all Rx mappings in FHIR mapping YAML files.
3. Rerun FHIR PIT on all datasets.
4. Redeploy the APIs.

OR

1. See if there's a shortcut we can identify to replace (2) and (3) in the above plan.

Rx variables:
https://mor.nlm.nih.gov/RxNav/
https://uts.nlm.nih.gov/uts/umls/vocabulary/RXNORM/6918