Make TerminologyCache return non-mutable ValidationResult and ValueSetExpansionOutcome

[dotasek]

As noted in #1700, results retrieved from the TerminologyCache can be mutated. This should never be the case. Though the validation code may need to alter a ValidationResult or ValueSetExpansionOutcome, it must do this in a way that does not change the cached object, which can impact future validations.

Making immutable versions of ValidationResult and ValueSetExpansionOutcome could make the code safer by throwing exceptions where returned objects are modified.

[grahamegrieve]

there's no way to do this other than manually, right? And what other things do we cache where we need to worry about this?