...designations in FhirCodeSystemLookupConverter: add separators to
returned language tags if needed

...language code in their "language" property; move language reference
set information ot the "designation-use-context" extension in
SnomedFhirCodeSystemLookupConverter.

...implicit value sets, leave the expansion step to a dedicated
SnomedFhirValueSetExpander class. This allows us to include the same
level of information for designations as in SNOMED CT lookup operations.

...FhirCodeSystemLookupRequest before passing it to a native request

...FhirCodeSystemValidateCodeRequest before passing it to a native
request

...parameter in FHIR operation controllers; the property is marked as
optional and the default value will be filled in when the request
corresponding to the operation is executed.

- SnomedDescription instances might not have the "languageCode" field
populated (a similar code snippet exists in
SnomedConceptSearchRequestEvaluator)
- The new FHIR model creates empty instances in some places where the
previous model returned null; the fix is to use "hasXYZ()" instead of
"getXYZ() == null" when checking the presence of a property.

...extensions in CodeSystem$lookup and ValueSet$expand test cases