Support predefined class-, property and enum mappings

[ewoutkramer]

At this moment, the only way to initialize a ModelInspector is to reflect on the types for the POCOs. This is normally fast enough, but there are certainly other options, like pre-generating the metadata that is normally reflected, possibly at the same moment that you generate the POCO's. To enable this scenario, we have to open up the public surface of ModelInspector, ClassMapping, PropertyMapping and EnumMapping a bit, so source code generators are also able to initialize a ModelInspector.

This PR fixes #2794. Thanks to @almostchristian for bringing this usecase up, and working on its design.

[ewoutkramer]

Ok, I have made a first version for @almostchristian to review. Note that I did not use required properties in the end, the few things that are required are still arguments to the constructor. In any case, all of that is now public/init.

I did not want to make the constructors on ClassMapping/PropertyMapping public, since I have other plans with them, but I created subclasses PocoClassMapping/PocoPropertyMapping/PocoEnumMapping that do, so that's what you should use. And maybe it will turn out we won't need it, then we can remove it again.

Anyway, Christian, if you're happy with it, we can pull it, and then there will be a new alpha release built automatically that you can test as well.

[mmsmits]

@almostchristian Is this something you are willing to take a look at?

[almostchristian]

From what I see in the firely sdk codebase, no code seems to be using the PropertyMapping.NativeProperty field. Searching in Github, I see it is being used in the validator repo by @brianpos https://github.com/brianpos/Hl7.Fhir.FhirPath.Validator/blob/6684bdd8fc19626be05531b8d75eb60e2d7b757e/src/Hl7.Fhir.Base.FhirPath.Validator/BaseFhirPathExpressionVisitor.cs#L672

[mmsmits]

@almostchristian Do you mind reviewing this PR for us? Would really appreciate it.