This introduces a new fs.Option flag, Sensitive and uses this along
with IsPassword to redact the info in the config file for support
purposes.
It adds this flag into backends where appropriate. It was necessary to
add oauthutil.SharedOptions to some backends as they were missing
them.
Fixes#5209
Before this change we were putting connections into the connection
pool which had a local context in.
This meant that when the operation had finished the context was
cancelled and the connection became unusable.
See: https://forum.rclone.org/t/failed-to-sync-context-canceled/34017/