Preprints authors should be able to attach supplementary files

Is your feature request related to a problem? Please describe.
As a preprints author, I would like to be able to attach supplementary files to my repository submission, similar to how journal authors can.

As a repository administrator, I would like to customize the text related to supplementary files during the submission process.

Each supplementary file should be able to have its own corresponding DOI.

Describe the solution you'd like
Preprint authors and moderators can attach supplementary files to their submissions. The supplementary files can be uploaded and stored in the same database. Each supplementary file should have its own DOI (if enabled by admin).

For Repository admin, this should be a toggle-able option: supplementary files as attachments or supplementary files. There should also be a toggle option after "Supp file as attachments": DOI for supp files.

For Repository admin, there should be an option to provide custom text that would be displayed alongside the supplementary file attachment portion of the workflow, potentially with additional information in a pop-up modal.

Bonus: It would be nice to be able to add a question about supplementary files (rather than having the question as a custom metadata request which appears on the main submission page).