Implement support for clientid and secret parameters to authorize endpoint

[dniel]

To be able to authenticate CLI or daemon clients to using traefik-forward-auth0 it need to support the client credentials grant authentication flow. .

Example of use cases:

• maven cli need to call nexus http backend to download/upload artifacts.

[dniel]

delayed, but not forgotten.
Need to stabilize the application first some more before adding more features.