This PR removes the `OAuthContextSettings` class, and makes the `--oauth-url` option available on each backend that uses it, so it can be configured as part of the destination.
To assemble everything related to configuring OAuth, the code was moved to the `AuthIdOptionsHelper`, such that the usage can be shared between implementations.
This PR also adds the option to set the default OAuth url from the environment variable `DUPLICATI_OAUTH_SERVICE`.
The server can then be set in the following locations (most important last):
- Environment variable
- Server-wide advanced settings
- Backup job advanced settings (or commandline)
- Destination url
Carl Johnsen