Implement a remote term service provider using a REST client

[JohnTimm]

We now have a registry-based terminology service provider (RegistryTermServiceProvider) and a graph-based terminology service provider (GraphTermServiceProvider). If we would like to integrate an existing terminology service (e.g. SNOMED-CT endpoint, LOINC endpoint, ontoserver, etc.), then we will need to implement a remote term service provider (RemoteTermServicdeProvider) that uses a REST client to forward requests to another server.

[kmbarton423]

Confirmed successful lookup on SNOMED-CT

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added the following to fhir-server-config.json (same level as "core"):

    "term": {
         "remoteTermServiceProviders": [{
            "__comment": "Configuration for public SNOMED-CT endpoint",
            "enabled": true,
            "base": "https://snowstorm-fhir.snomedtools.org/fhir",
            "supports": [{
                "system": "http://snomed.info/sct"
            }]
        }]
    },

added trustDefaultCerts="true" to server.xml:

GET https://localhost:9443/fhir-server/api/v4/CodeSystem/$lookup?system=http://snomed.info/sct&code=195967001