Add a proxier that does what the bouncer does, but proxies requests instead of bouncing them.

The current bouncer tests are more end-to-end tests. We should keep these but also add tests for the individual bouncer request handler methods.

See #1 for the initial bouncer work.
Yes, for two reasons:
  - As mentioned above `default` isn't working
  - `get` is given for `BounceAuthHandler`, which `ProxyAuthHandler` subclasses

Happy to try out alternatives if you have suggestions.