Add support for default version of registry resources

[JohnTimm]

FHIRRegistryResource.getResource(String url) currently parses the url parameter as a canonical url (<url>|<version>). If the version is not present, then it will look for all versions with the given url across all registry resource providers. It will then dedup/sort this list and return the "latest version". In addition to "latest version" there is a notion of "default version". The "default version" may be designated by an implementation (see TerminologyCapabilities.codeSystem.version.isDefault for an example of this).

We should make the following changes to support the idea of a "default version".

• Add a defaultVersion field to the FHIRRegistryResource class along with constructor(s) that include defaultVersion as a parameter and an isDefaultVersion method
• Add factory methods that create FHIRRegistryResource instances and allow for a defaultVersion to be specified
• Add unit tests to test "default version" and "latest version" behavior
• Update FHIRRegistry, PackageRegistryResourceProvider and ServerRegistryResourceProvider to use "default version" when a version is not specified

If no version is designated as the "default version", then the logic falls back to "latest version" based on sort order. If multiple versions are designated as the "default version" then the first one encountered will be used.

We also discussed the potential for supporting a defaultVersion field in the .index.json NPM package format we use to load resources for a PackageRegistryResourceProvider but decided it was lower priority.

We may want to consider an extension that allows a client to specify that a particular definitional resource (e.g CodeSystem, StructureDefinition, etc.) has a business version that is intended to be the default version. We could then update FHIRRegistryResource.from method to use that extension (if it exists).

[tbieste]

I verified in my dev workspace that if the FHIRRegistryResource.from method sets default to true for a CodeSystem version, then the <base>/metadata?mode=terminology API shows isDefault: true for that CodeSystem version. More complete QA of the functionality can be done when there is a way for a client to indicate the default version of definitional resource via an extension is implemented in the future.