[FhirPath] more customization in fhirpathcompilercache extension methods

[Kasdejong]

The fhirpath predicate/istrue/select/etc extension methods take an evaluationcontext as a parameter, but:

• It is not easy to quickly set environment variables like this
• they do not set %resource, %rootresource and %context

We should revise this, since currently this is only possible in our lower level API. This might be breaking.