Before this change any backends which required extra config in the
oauth phase (like the `region` for zoho) didn't work with `rclone
authorize`.
This change serializes the extra config and passes it to `rclone
authorize` and returns new config items to be set from rclone
authorize.
`rclone authorize` will still accept its previous configuration
parameters for use with old rclones.
Fixes#5178
f52ae75a51