Feature Request: show hierarchy of collections via API

https://dataverse.harvard.edu/dataverse/airqualregs has a hierarchy of collections underneath it that looks something like this:

However, you'd never discover this tree-like structure from looking by looking at the Dataverse UI. You see that there are 12 collections. That's it.

The Dataverse API doesn't expose the hierarchy of collections either. It could be interesting to create an API for this. A UI can come later, in the new SPA.

I'll size this as a 20 for now.

p.s. The dataviz above comes from https://github.com/IQSS/dataverse/issues/5603#issuecomment-470993028 and https://github.com/IQSS/dataverse/issues/2155#issuecomment-1269189972

Some other conversations along there lines:

• Dataverse 4 (collections): https://groups.google.com/g/dataverse-community/c/V1W2-OGH6po/m/7MiPR4jlnRUJ
• Implementing some form of folder structure: https://groups.google.com/g/dataverse-community/c/R88pRTlfD30/m/vwEKdWV3BQAJ

Start by reviewing the existing collection API and the linked discussions about collection hierarchy and folder structure. Before implementation, define the API endpoint, response representation, and expected behavior for nested collections; done means the hierarchy can be retrieved programmatically without requiring a UI.