implement --cacert option advertised by rustic_server

[thkoch2001]

The documentation of rustic says:

you can pass rustic the certificate filename via the --cacert option

As does the USAGE documentation of rustic_server:

To access this server via restic use --cacert public_key

But there is no such option. There is such an option in restic and the last snippet was copied from the restic documentation.

I understand from https://github.com/rustic-rs/rustic/issues/679 that there seems to be a workaround, probably by copying the self-signed certificate in some system location.

Also when implementing this functionality, it would be good to also expose it from the rustic configuration file.