Add Source generation for custom resources

My company is looking into using more and more custom FHIR resources, So I'm looking to develop a source generator to auto generate the resource code based on the structure definition. My idea is that it will look for files with a name that ends with `.StructureDefinition.json` and then generate code from those files.

I tried my hand in this but I think this could work.

I'm currently working on this branch:
https://github.com/almostchristian/fhir-codegen/tree/feature/sourcegen

Some issues that I'm looking to resolving:
- ~~change namespace to be the same as the `.` instead of the constant namespace. This should be a simple change.~~ supported in latest change
- ~~nested enum and generic `Code` property generation, I'm still trying to figure out how to get this to work.~~ supported in latest change