feat: Extend getCollectionItems dataset search functionality

[vera]

What this PR does / why we need it:

This PR is our proposal for extending the getCollectionItems use case to expose more of the /api/search API’s dataset search functionality while preserving the existing API behavior.

• Optionally include collections (show_collections)
• Optionally include metadata blocks (metadata_fields)
• Support complex filter queries, which should be passed through unchanged

See #489 for more details.

I'm marking it as a draft PR for now. It still needs tests too.

Which issue(s) this PR closes:

• Closes Feature Request: Extend getCollectionItems dataset search functionality #489

Related Dataverse PRs:

• None

Special notes for your reviewer:

None

Suggestions on how to test this:

TODO

Is there a release notes or changelog update needed for this change?:

Yes (TODO)

Additional documentation:

None