Support customizable compartment definitions

[lmsurpre]

Is your feature request related to a problem? Please describe.
Compartment search provides a way to efficient scope a search request to resources that have a single parameter value of some kind (e.g. a specific patient id).

It might be nice for users (i.e. operators of a given fhir server deployment) to be able to both:

1. customize existing compartments (like the patient compartment) with their own "inclusion criteria" for what to include/exclude; and
2. define new compartment definitions that can be used to scope search requests by a common criteria

Describe the solution you'd like

1. Update CompartmentUtil to pull compartment definitions from the registry (instead of a packaged compartments.json).
2. Document how users/operators can override the default compartment definitions.
3. Document how users/operators can supply new compartment definitions.

Describe alternatives you've considered
Do not support customizable compartment definitions.

Acceptance Criteria

Additional context
This feature becomes more powerful with #2371

[lmsurpre]

I found that the CompartmentDefinition.code has a required binding of Patient | Encounter | RelatedPerson | Practitioner | Device.
Therefor, to enable custom compartment definitions, I introduced a special extension with a url of http://ibm.com/fhir/extension/custom-compartment-type.

Using this extension, I registered a new compartment definition for "DrugPlan" as follows:

[{
    "resourceType": "CompartmentDefinition",
    "url": "http://ibm.com/health-interop/fhir/CompartmentDefinition/drugPlanCompartment",
    "name": "DrugPlan",
    "status": "draft",
    "_code": {
        "extension": [
            {
                "url": "http://terminology.hl7.org/CodeSystem/data-absent-reason",
                "valueCode": "not-permitted"
            },
            {
                "url": "http://ibm.com/fhir/extension/custom-compartment-type",
                "valueCode": "DrugPlan"
            }
        ]
    },
    "search": false,
    "resource": [
        {
            "code": "List",
            "param": [
                "planid"
            ]
        },
        {
            "code": "MedicationKnowledge",
            "param": [
                "DrugPlan"
            ]
        }
    ]
}]

With this CompartmentDefinition and the corresponding search parameter available in the registry, I can now perform compartment searches like GET [base]/DrugPlan/123/List or GET [base]/DrugPlan/123/MedicationKnowledge and the results appear as expected.
Additionally, it becomes possible to scope a request to more than one drugplan via persistence interceptor.

[lmsurpre]

added very basic documentation at #2469
calling this one good