Support "CQL light" - x-fhir-query plus extensions in questionnaire

[ewoutkramer]

The SDC IG defines capabilities to populate Questionnaires using CQL. Although powerful, it is also possible to use FhirPath + additional extensions to achieve much of the same goals. FhirPath population uses three ingredients

• An extension comparable to the "context" of CQL
• An extension using x-fhir-query to serve as a kind of retrieve
• An expression in FhirPath.

This is described in more detail here: https://hl7.org/fhir/uv/sdc/STU3/populate.html#expression-based-population

It is likely that, at least for the near-future IGs, this system is used instead of full CQL. We should take a look at it and see how we can support it. One possible way to do it, is by translating the FhirPath + the extensions to ELM itself, and then running it using our CQL SDK.