Provide attributes for storing values relevant for XDS Metadata Update

[unixoid]

Closes #

📑 Description

This PR introduces changes which enable a mapping between MHD DocumentReference resources and XDS Metadata Update requests. The following XDS attributes are in focus:

• logicalID
• repositoryUniqueID
• version
• documentAvailability

The first one is mapped to a dedicated slice of DocumentReference.identifier, the latter three -- to extensions.

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed
• I have selected a committee co-chair to review the PR

ℹ Additional Information

[JohnMoehrke]

Committee discussion:

There needs to be an Issue created that explains why these changes are needed. Please explain the Use-Case, and why this can't be solved without a change to IHE-MHD.

Would support for these elements be needed by all MHD actors? Or would there be an named Option defined for support of these elements? Given that the majority of MHD actors will never need these elements, it seems not appropriate to require support for these elements on all Actors.

When is it allowed to have these elements populated? Are they allowed to be populated in ITI-65? I would say not.

The scale of this change will need to be done with a CP. Having the Issue and PR available is a benefit to the CP process. We need the CP process for the sake of governance and visibility.