Replace specification.zip with FHIR npm package(s)

[mmsmits]

Having the FHIR definitions in specification.zip isn't necessary anymore since the introduction of FHIR packages (npm).
We should replace all functionality around specification.zip with either static npm packages or package resolvers.

More details about how to implement such a functionality and where it should live, is to be discussed below.

Tasklist:

• Add package library to SDK
• Implement NpmPackageResolver
• Implement version specific CorePackageResolver
• Use specific xml package for .xsd files.
• Nullability
• Move NpmPackageResolver to Support.Poco

[mmsmits]

Some options:

• Resolve packages live. (Much more support when users have firewall issues, and have to be online)
• Use a Nuget package to supply the fhir packages.
• Embed packages in the solution (How is that different from specification.zip)

[ewoutkramer]

And we would like people to be able to add their own/their SDO/their companies packages too. So, this should then be a matter of dropping npm files somewhere (this is what the tools produce). This seems to make using NuGet less favorable, otherwise everyone needs to wrap their NPMs in Nugets.

[ewoutkramer]

We should probably package the npm packages with the SDK, and add a feature to have additional npm packages installed on your machine's npm cache (which you can manage with any npm client - or torinox)