Enhancement: Add config option to set SFTP command arguments

The `sftp.args` option can be passed to restic (using `-o`) to specify
custom arguments to be used by the SSH command executed by the SFTP
backend.

Before this change, a common scenario where a custom identity file was
needed for the SSH connection, required the full command to be
specified:
`-o sftp.command='ssh user@host:port -i /ssh/my_private_key -s sftp'`

With this new configuration option:
`-o sftp.args='-i /ssh/my_private_key'`

https://github.com/restic/restic/pull/4519
https://github.com/restic/restic/issues/4241