Support rest backend credentials from environment

[xanderio]

Restic support getting username and password from environment variables, RESTIC_REST_USERNAME and RESTIC_REST_PASSWORD respectively. https://restic.readthedocs.io/en/stable/030_preparing_a_new_repo.html#rest-server

[magrathean-uk]

I opened a focused draft PR for this: #1847

It adds RESTIC_REST_USERNAME and RESTIC_REST_PASSWORD support for REST backend credentials.