Support reference resolve

[juliangruendner]

The fhir data evaluator should extended to support the resolving of references, so that the following fhir path is correctly resolved to extract the object for stratification:

Condition.subject.resolve().ofType(Patient).gender

MedicationAdministration.medication.resolve().ofType(Medication).code.coding.where(system="http://fhir.de/CodeSystem/bfarm/atc")

Example data for testing, see:
https://github.com/medizininformatik-initiative/feasibility-deploy/blob/main/feasibility-triangle/get-mii-testdata.sh