Is your feature request related to a problem? Please describe.
The file
https://github.com/pnp/powershell/blob/dev/src/Commands/DocumentSets/AddDocumentSet.cs

Uses the root folder of the document library and does not accept a folder as a parameter.

Describe the solution you'd like
Add a folder parameter like the add-pnplistitem command.

Describe alternatives you've considered
Probably need to manually use the SP client api awkwardly.

Additional context
I'm trying to make document sets within folders in a document library.