add synthetic data source options

from @andinet :

Synthea is a synthetic EHR data generator

https://github.com/synthetichealth/synthea

It can export data in HL7 FHIR format.

Here is the original paper that talks about the simulator
https://academic.oup.com/jamia/article/25/3/230/4098271

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more comments by me:

Synthea does not create a fhir server for you, but rather can write json to your disk that looks exactly like what you'd get if you looked up patient data that was served via the FHIR protocol ([see here](https://github.com/synthetichealth/synthea/wiki/HL7-FHIR)). It's as though you had a FHIR server with the data in it, queried the server to get some json representations of the FHIR resources, and then saved that text json to disk. So the tool we are creating still has value added beyond what Synthea does, because our tool sets up a containerized server for you. However, our tool is less ambitious than Synthea in that we are (currently) only supporting Patients and Observations (compare to the giant list of FHIR resources seen [here again](https://github.com/synthetichealth/synthea/wiki/HL7-FHIR)).

So in conclusion I do believe it will be valuable, perhaps later, to incorporate synthea as one form of data generation that our tool can use.

Start by reading the Synthea repository, its HL7 FHIR documentation, and the linked paper to understand the proposed data source. The issue does not name project files, tests, an entry point, or a concrete completion criterion, so the intended integration scope would need to be clarified before implementation.

Written by the indexing model from the issue text.