Add source generation

[almostchristian]

• Added source generation package Microsoft.Health.Fhir.CodeGeneration.
• Updates shared libraries to multitarget netstandard2.0 which is required for source generators

Usage:

[GeneratedFhir("Models/Pokemon.StructureDefinition.json",
   TerminologyResources = new[] { "Models/PokemonType.CodeSystem.json", "Models/PokemonType.ValueSet.json" })]
public partial class Pokemon
{
}

Currently the FhirVersionInfo used for code generation is hard-coded to R4B, and I don't see any difference when it's R5.

Resolves #140

[ewoutkramer]

@GinoCanessa - Hadn't seen this PR before, but I guess this needs to change quite a bit after the refactoring of the codegen to use StructureDefinition instead of the custom metadata structures....

[almostchristian]

Most of my code changes is related to multitargeting to netstandard2.0 which is a requirement for source generators. But from what I saw from the experimental branch, this will not be simple.

[GinoCanessa]

Hi Christian - it is on my list to re-review everything. I started looking into it yesterday and will be making sure your use case is not left out.

[GinoCanessa]

Alright @almostchristian , the current contents of the experimental branch should have all libraries correctly multi-targeting both .Net 8 and NetStandard 2.0. Note that you need a C# 12.0-compatible compiler to build them*. Please test and let me know if there are any issues.

*After we merge into main, I will finish sorting out the pipeline to release on NuGet as libraries and the CLI as a tool, so the build requirements are only until then.

Cheers!