Integrate with Traefik, Nginx, Ambassador, Envoy

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

I'd like to use Ory Oathkeeper with:

• Traefik Forward Auth: (example)
  • Implementation is required
  • Document set up (with example config) in Oathkeeper -> Scenarios (new section!) ->Nginx ForwardAuth
• Nginx Subrequest:
  • Implementation should already work with the existing decisions endpoint
  • Document set up (with example config) in Oathkeeper -> Scenarios (new section!) ->Nginx ForwardAuth
• Ambassador Ambassador AuthService Plugin:
  • Implementation should already work with the existing decisions endpoint
  • Document set up (with example config) in Oathkeeper -> Scenarios (new section!) ->Ambassador AuthService Plugin
• Envoy External Authorization
  • Research is needed to decide if a custom endpoint is required or not
  • Document set up (with example config) in Oathkeeper -> Scenarios (new section!) -> Envoy External Authorization

Additional info

As per my comment https://github.com/ory/oathkeeper/pull/265#issuecomment-534967157 we would like to offer one endpoint per service:

• /decisions/generic/... (this is currently /decisions)
• /decisions/traefik/...
• ...

For backwards compatibility ideas see https://github.com/ory/oathkeeper/pull/265/files#r329310302

For implementation ideas see this PR: https://github.com/ory/oathkeeper/pull/265

Context

See comment https://github.com/ory/oathkeeper/issues/263#issuecomment-605912334

Review PR 265 and Oathkeeper’s existing /decisions endpoint first, then compare the listed Traefik, Nginx, Ambassador, and Envoy integration requirements. Done means the required service-specific integration behavior is defined or implemented and setup examples are documented under docs/oathkeeper in the proposed Scenarios sections.