Implement CQL ValueSet Retrieve

[alexanderkiel]

Retrieve Expressions in CQL should work with ValueSet references. The ValueSet should be expanded during query compilation time into a list of codes. That codes will be used in the database search clause like multiple code were already possible.

Example

library "q66-icd10-value-set"
using FHIR version '4.0.0'
include FHIRHelpers version '4.0.0'

valueset icd10_e10: 'http://fhir.org/VCL?v1=(http://fhir.de/CodeSystem/bfarm/icd-10-gm)concept<<E10'

context Patient

define InInitialPopulation:
  exists [Condition: icd10_e10]

Recent VCL support implemented in #3275 comes in handy here.