MCR-3600 Turn MCRXMLMetadataManager into an interface

[toKrause]

Link to jira.

Pull Request Checklist (Author)

Please go through the following checklist before assigning the PR for review:

Ticket & Documentation

• The issue in the ticket is clearly described and the solution is documented.
• Design decisions (if any) are explained.
• The ticket references the correct source and target branches.
• The fixed-version is correctly set in the ticket and matches the PR's target branch (main).

Feature & Improvement Specific Checks

• Instructions on how to test or use the feature are included or linked (e.g. to documentation).
• For UI changes: before & after screenshots are attached.
• New features or migrations are documented.
• Does this change affect existing applications, data, or configurations?
  • Yes: Is a migration required? If yes, describe it.
  • Breaking change is marked in the commit message.

MyCoRe-Org/mycore-website#130

Testing

• I have tested the changes locally.
• The feature behaves as described in the ticket.
• Were existing tests modified?
  • Yes: explain the changes for reviewers.

Test now configure and use an MCRXMLMetadatamanager instead of an MCRXMLMetadataManagerAdapter,

MCR Conventions & Metadata

• MCR naming conventions are followed
• If the public API has changed:
  • Old API is deprecated or a migration is documented.
  • If not, no action needed.
• Java license headers are added where necessary.
• Javadoc is written for non-self-explanatory classes/methods (Clean Code).
• All configuration options are documented in Javadoc and mycore.properties.
• No default properties are hardcoded — all set via mycore.properties.

Multi-Repo Considerations

• Is an equivalent PR in MIR required?
  • If yes, is it already created?