Implement $expand operation

**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](https://www.hl7.org/fhir/codesystem.html)
[ValueSet](https://www.hl7.org/fhir/valueset.html)
[$expand](https://www.hl7.org/fhir/valueset-operation-expand.html)

Start by reading the linked FHIR CodeSystem, ValueSet, and $expand specifications, then trace the ValueSet/123ABC/$expand request entry point in the service. The work is done when the operation returns all codes from the CodeSystems referenced by the requested ValueSet, with the initial base URL form supported.

Written by the indexing model from the issue text.