[FhirPath] Add memberOf() function.

[ewoutkramer]

Is your feature request related to a problem? Please describe.
The us core profiles (e.g. http://hl7.org/fhir/us/core/StructureDefinition/us-core-condition) contain invariants using the memberOf() function (an extension on the standard FhirPath functions, see here https://hl7.org/fhir/fhirpath.html#functions),

Describe the solution you'd like
We should implement these additional functions, as much as possible, to make sure the .NET library is useable with these profiles.

[ewoutkramer]

We have a "we don't support this function" quickfix now as a PR: FirelyTeam/firely-net-common#105

[marcovisserFurore]

Discussed in the standup today:

• The EvaluationContext of FhirPath should be extended with a ITerminologyService.
• The function memberOf() should then take the valueset (argument of memberOf) and check whether the input is a valid code in the valueset (ITerminologyService.ValueSetValidateCode()) or not.

[cheng13231]

I noticed that memberOf() function is used in CARIN profiles as well. For example, it is used in following constraints:

• EOB-institutional-item-or-header-adjudication
• EOB-careteam-qualification
• EOB-prof-all-transportation-supportinginfo-linked-to-line

[marcovisserFurore]

See also #2404

[marcovisserFurore]

Resolved by PR #2450