Implement $expand operation

[holpit]

User story
As a user of HL7 FHIR Version 4, I want to use the $expand operation so that I can use CodeSystem and ValueSet to manage the "CodableConcept" for all resources.

Acceptance criteria

1. Create one or more CodeSystem
2. Create one or more ValueSet
3. Each ValueSet has a reference to one or more CodeSystem
4. When I use the $expand operation on a ValueSet, than the system returns all the CodeSystem's codes referenced in the ValueSet.

The implementation can be incremental. The base command [baseURl]/ValueSet/123ABC/$expand is enough for start.

Details can be found here:
CodeSystem
ValueSet
$expand

[hansenms]

This is a good suggestion. It links to terminology services work, e.g. #810 and some others.

[CaitlinV39]

AB#85021