Route requests to sidecar container first

[edorizzardi]

In my organization, we require to route requests to oauth2proxy to its sidecar container first. At the moment, is not possible to achieve this with the helm chart, since the service's targetPort is by default httpSchema. I propose some non-breaking changes to allow for this scenario, introducing the new .Values.service.targetPort.

Pull request: #270

[pierluigilenoci]

Thank you for your contribution. 🚀