Feature Request/Idea: UI support for nested compound fields

Overview of the Feature Request

Currently, Dataverse doesn't fully support nested compound fields (i.e. compound fields within compound fields). More precisely, the UI cannot display them, although the API and the internal data model already appear to support them. In some constellations, the SOLR schema is also not updated correctly.

What kind of user is the feature intended for?

Any user using the UI to view/edit datasets (curator, depositor, guest, superuser...) and API users

What inspired the request?

We would like to use custom metadata schemas that contain nested compound fields.

DataCite v>=4.1 also contains fields like this, e.g. see "7. Contributor" (0-n) with its children "7.4 nameIdentifier" (0-n) and "7.5 affiliation" (0-n) in https://schema.datacite.org/meta/kernel-4.4/doc/DataCite-MetadataKernel_v4.4.pdf

I have tested the following three basic scenarios of nested compound fields (load custom TSV, upload dataset via API, export dataset via API):

1. compound field nested within compound field
2. like 1. but outer field has allowmultiple TRUE
3. like 1. but both fields have allowmultiple TRUE

The TSV files and JSON datasets are here: nested_compound_test.tar.gz

Although the UI cannot display the nested fields, in scenarios 1 and 3 the export actually matched the initial upload, thus it seems the API and the internal data model already support this feature. In scenario 2 I got an error message at export (Exception thrown from bean: org.apache.solr.client.solrj.impl.HttpSolrClient$RemoteSolrException: Error from server at http://solr:8983/solr/collection1: ERROR: [doc=dataset_9_draft] multiple values encountered for non multiValued field). After manually editing the SOLR schema file to set multiValued="true", the export succeeded here as well.

What existing behavior do you want changed?

The SOLR schema should be updated correctly when a TSV with nested compound fields is uploaded.

Any brand new behavior do you want to add to Dataverse?

The UI should be able to display nested compound fields when creating, editing and viewing a dataset.

Currently, it looks like this (empty because inner children aren't shown):

Any related open or closed issues to this feature request?

https://github.com/IQSS/dataverse/issues/377 is related. When allowing multiples of a compound child, in some cases the compound child itself should also be a compound, e.g.:

"Author" is a compound field. To allow an author to have multiple identifiers, the children "Identifier Scheme" and "Identifier" should be grouped in a repeatable nested compound field.

(See also "7. Contributor" (0-n), "7.4 nameIdentifier" (0-n), "7.4.a nameIdentifierScheme", "7.4.b schemeURI" in https://schema.datacite.org/meta/kernel-4.4/doc/DataCite-MetadataKernel_v4.4.pdf (present since DataCite v4.1 as property 7.5))

cc @johannes-darms

Start by reproducing the three nested-compound scenarios using the linked nested_compound_test.tar.gz files, then trace the dataset UI rendering/editing flow and the SOLR schema update path. Done means nested compound fields display correctly when creating, editing, and viewing datasets, and TSV uploads generate a schema that exports successfully without the reported multiValued error.