fhir: map primitive elements

[hunterachieng]

Summary

Add ability to map primitive elements in fhir by focusing on elements starting with an _.
Regenerated fhir-eswatini with tests to check behavior

• Ensured we do not throw away primitive elements in generate-schema then mapped by checking if the parent has an extension.

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[josephjclark]

Maybe a bit of confusion over whether we're handling primitve types or primitive extensions? I'm expecting that primitive types work already, and we're just adding support for extensions here