This is a very large change which turns the post Config function in
backends into a state based call and response system so that
alternative user interfaces can be added.
The existing config logic has been converted, but it is quite
complicated and folloup commits will likely be needed to fix it!
Follow up commits will add a command line and API based way of using
this configuration system.
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
This splits config.go into ui.go for the user interface functions and
authorize.go for the implementation of `rclone authorize`.
It also moves the tests into the correct places (including one from
obscure which was in the wrong place).
