Use environment variable for setting "our" configuration
We previously exposed the authOptions.extraArgs value which allows users to specify arbitrary configuration options for oauth2-proxy. However, when a user specifies a config option that we previously specified, their setting will simply be ignored.
Since oauth2-proxy can read it's configuration from multiple sources, we can use environment variables to specify our "default" configuration. Since envrionment variables have a lower precedence than command line arguments, the users can now overwrite settings that we specify.
Unfortunately, it makes our unit tests a bit more verbose.