fhir v4 adaptor

[josephjclark]

-- stub issue--

Build out a full canonical auto-generated fhir-v4 adaptor

Include basic fhir verbs

Include hand-made and unit-tested datatype builders

Focus on a really good builder API

Think about how we'd build an OpenFn adaptor an an open-source builder library

This should be suitable for rebasing both ndr-et and fr onto. But don't worry too much about that side of things.

Try and get all resources built, with an init function. Don't worry about extensions and other IGs too much