Support the HL7 Terminology Registry

[ewoutkramer]

HL7 introduced a Terminology Registry (https://github.com/FHIR/ig-registry/blob/master/tx-registry-doco.md), which holds a list of terminology servers and can be used to determine which terminology server to reach out to for which valuesets. This allows a validator to reach out to a country's terminology server to get localized terminologies.

Currently, the SDK allows you to hard-code one or more servers, including routing for specific valuesets. I could see how our MultiTerminologyService can be configured from the terminology registry. Or maybe there are other smart things we should do.

So, we need to figure out how to support this in a sensible way.