Allow users to provide their own S3/B2 bucket and credentials

### Describe the problem to be solved

Since a big part of the expense of running a PeerTube instance is just storing the files, it would be nice if instance admins could provide a way for users to provide their own block storage bucket for their videos, so that users can foot their own storage bill without having to do all the other difficult parts of running an instance themselves.

### Describe the solution you would like

The user profile could have a place to optionally put the following information for Bring-Your-Own Storage:

* API Endpoint (with some easily selected defaults for S3, B2, etc.)
* Bucket ID
* Token and secret

No files, tests, or entry points are named. Start by locating the user profile settings and existing video storage configuration, then determine how optional S3/B2 endpoints, bucket IDs, tokens, and secrets would be validated and used; done means users can supply their own storage without affecting instances that use default storage.

Written by the indexing model from the issue text.