FHIR Interface Support

[GinoCanessa]

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Is your feature request related to a problem? Please describe.

With the release of FHIR R5, we have references to 'interfaces', but I cannot find a good way to resolve them in the SDK. For example, the $current-canonical operation is rooted in CanonicalResource, but I do not see a listing of which resources implement CanonicalResource. Thus, I need to manually look up the information in the spec in order to determine which resources should have the operation.

This probably fits as part of a larger discussion on what should be done with the FHIR interfaces in general.

Describe the solution you'd like

For the first part: an annotation in ModelInfo, a property/annotation in the POCOs, and/or a dictionary to lookup which resources map to which FHIR interfaces.

For the second part: decision on the C# class/interface structure that would allow more general use (e.g., .AsCanonicalResource() or (CanonicalResource) casting. I believe any implementation will be a little convoluted because the FHIR interfaces allow things like not specifying an element in a resource that is in the interface. I have some exploration code from when the changes to the FHIR specs were being discussed in this gist.

Describe alternatives you've considered

Manual lookup and tracking by implementers using the SDK.

Additional context
Cheers!