fix: fix bug in OAI-ORE exporter related to nested compound fields

[vera]

What this PR does / why we need it:

This PR enables the OAI-ORE exporter to generate a metadata export for metadata containing nested compound fields (i.e. compound fields within compound fields).

Which issue(s) this PR closes:

• Closes OAI-ORE exporter fails for too deeply nested metadata #10809

Special notes for your reviewer:

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Suggestions on how to test this:

1. Create a metadata block containing nested compound fields, e.g. use one in here: nested_compound_test.tar.gz

   curl http://localhost:8080/api/admin/datasetfield/load -H "Content-type: text/tab-separated-values" -X POST --upload-file nested_compound_test2.tsv

2. Enable metadata block

3. Create a dataset using the metadata block, e.g. using a JSON file in the archive linked above

   curl -H "X-Dataverse-key: $API_TOKEN" -X POST "http://localhost:8080/api/dataverses/root/datasets" --upload-file nested_compound_test2.json -H 'Content-type:application/json'

4. Go to http://localhost:8080/dataset.xhtml?persistentId=... (using PID returned by the API in 3.)

5. Publish dataset

6. Reload page

7. Open "Metadata" tab, click "Export Metadata" button and select "OAI_ORE"

8. Check that the export contains no errors: nested_compound_test2_oai_ore_export.json

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

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

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Additional documentation:

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