fhir-fr: Extensions

[josephjclark]

Summary

I wanted a quick look at adding better extensions to fhir-fr.

At the moment we're not even recognising extension properties - because of how they're encoded they're just ignored.

This PR improves things a little, but doesn't have any intelligence. Really, we need to:

• When parsing an extension which is defined by another structure, go and find that structure definition
• Generate code to create to add an extension properly to the resource, mapping the URL properly

Patient Example

The example is acutally quite complex because for some reason, birth-place and identifty-status are mapped very differently.

I need to better study the defintions and work out the mapping rules.

In fhir-ndr-et, we used the mappings file to manually map the extension key. But I really want to do better than that here.

This probably needs a good day's work to figure out.

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

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