Implement terminology-service FHIRPath functions via pluggable interface

[luisfabib]

Four FHIRPath functions defined in the FHIR supplement to the FHIRPath specification all raise
NotImplementedError unconditionally. These functions require either a TerminologyService
endpoint or access to the full StructureDefinition stack:

Function	Use case
memberOf(valueset)	Valueset membership check — appears in hundreds of built-in FHIR constraints
conformsTo(profile)	Profile conformance check
subsumes(code)	Terminology subsumption
subsumedBy(code)	Terminology subsumption (inverse)

Additionally, the error message in subsumedBy() incorrectly reads "subsumes() is not supported".

Proposed solution

Introduce a TerminologyService abstract interface (protocol) that the FHIRPath engine can
delegate to. Users register an implementation (wrapping a local FHIR server or a custom adapter)
via fhircraft.configure(terminology_service=...). When no service is registered, functions
raise a clear FHIRPathNotSupportedError with an actionable message instead of a bare
NotImplementedError.

Also fix

Correct the error message in subsumedBy() (additional.py ~L997) to reference subsumedBy()
instead of subsumes().