[Valdator] Support dependent profiles

[ewoutkramer]

HL7 has published a few extensions that we could implement and influence validation of profiles:

• HL7.FHIR.UV.EXTENSIONS\StructureDefinition Dependent Profiles - FHIR v5.0.0
  "Instances can only be valid against this profile, if they also successfully validate against the profile referenced by this extension."
• HL7.FHIR.UV.EXTENSIONS\StructureDefinition Compiles With Profile - FHIR v5.0.0
  "Asserts that instances that are valid against this profile are also automatically valid against the StructureDefinition referenced by this extension."

That last one looks like an optimization to me, and we already have this logic implemented in the new validator (if an instance is valid against a profile X, the instance is also valid against all parent profiles of X).

[ewoutkramer]

Operationally, I don't see what the results of that validation would do - except if you want to mark meta.profile with profiles against we have validated conformance. Then you can put both there.

The first one is basically saying: after finishing validation, continue with the profile given in the extension. I'd say just start a new validation and include the results in the end schema. We might want to double check that the locations and messages make sense if we have multiple profiles in use, but we do something like that when we validate meta.profile, an extension (which is another structdef) or a referenced profile (typeRef.profile), so it is not new.