Create a terminology service that validates local valuesets with external codesystems

[ewoutkramer]

Create a new ITerminologyService implementation that could do a $validate for a ValueSet, even if it does not have the codesystems mentioned in the valueset. This can be done by interpreting the composition rules in the ValueSet and then translating them to $lookups on a CodeSystem, basically bridging between a ValueSet (which the external server might not have) and the CodeSystem (which the external server could have).

That was Tim's idea, and I think we can make it work in some limited, but common cases. After studying the Java source code of the terminology client, we have concluded that Grahame is doing the same thing, in a very limited (but useful) way. We could copy that approach.