add mimic-iv on fhir data source

[ebrahimebrahim]

https://physionet.org/content/mimic-iv-fhir-demo/2.0/

Contains the data from 100 patients taken from MIMIC-IV, but organized in a FHIR-like format.
It's FHIR-like data exported to json files that you can download -- then it'd be up to us to serve the json as if one is reading a real FHIR server.
Would definitely be a good data source option to add to our fhir-sandbox software if we had time/interest later.