oauth2_client_credentials: Enable timeout configuration for calls to token endpoint

[catper]

Is your feature request related to a problem? Please describe.

When running on Kubernetes with oauth2_client_credentials we find that the call to the token endpoint often times out. The default timeout is 1 second, which is a long time and should be enough but we find that for our setup, it generally isn't.

Describe the solution you'd like
I'd like to be able to specify a timeout for the request to the token endpoint so that we are not hindered by things that really should be circumventable. Something akin to the retry config that you can set for oauth2_introspection should do the trick.

Describe alternatives you've considered

We could change httpx.NewResilientClientLatencyToleranceMedium to httpx.NewResilientClientLatencyToleranceHigh in pipeline/authn/authenticator_oauth2_client_credentials.go but going for an optional config option seems like a better idea.

Additional context

Add any other context or screenshots about the feature request here.