Be able to generate code/classes for any package, not just core specs

It would be super useful if `fhir-codegen` could generate classes from any FHIR package. Right now it is limited to the core packages, but Gino mentioned this is not too far off?

I have heard multiple customers asking for this, to get them started with implementing a specification with strong typing/validation.

Previously discussed at https://chat.fhir.org/#narrow/stream/179171-dotnet/topic/Microsoft.20fhir-codegen-cli/near/383002869