Enhancement: Make timeout for stuck requests customizable

Restic monitors connections to the backend to detect stuck requests. If a
request does not return any data within five minutes, restic assumes the
request is stuck and retries it. However, for large repositories this timeout
might be insufficient to collect a list of all files, causing the following
error:

`List(data) returned error, retrying after 1s: [...]: request timeout`

It is now possible to increase the timeout using the `--stuck-request-timeout`
option.

https://github.com/restic/restic/issues/4970
https://github.com/restic/restic/pull/5014
