Add domain for FHIR ObservationCategoryMaps

[ibacher]

See this comment for details about this PR's rationale.

[ibacher]

So, FHIR Observations have a category property that tries to cover "what kind" of observation that they are. In OMRS, the same concept is usually represented by "concept class" for the concept associated with the observation. This provides a way of mapping between the FHIR category field and the concept's concept class because, wherever possible, we want to use the standard FHIR value set, but OMRS's ConceptClasses don't necessarily always align with those categories.

Basically, we use this field to power the "lab results viewer" and could use it for other things, like vital signs, but we need a way of allowing custom mappings to accommodate this flexibly.

[ibacher]

I still need to move the actual logic into the FHIR2 module, so this remains pending.