Enhancement: Allow custom User-Agent to be specified for outgoing requests

Restic now permits setting a custom `User-Agent` for outgoing HTTP requests 
using the global flag `--http-user-agent` or the `RESTIC_HTTP_USER_AGENT` 
environment variable.

https://github.com/restic/restic/issues/4768
https://github.com/restic/restic/pull/4810