add support for route idp_client_id and idp_client_secret

[calebdoxsey]

Summary

Add support for route idp_client_id and idp_client_secret. This is done with an annotation like:

ingress.pomerium.io/identity_provider_secret: idp-secret

And a corresponding secret:

apiVersion: v1
kind: Secret
type: Opaque
data:
  client_id: ''
  client_secret: ''
metadata:
  name: idp-secret
  namespace: pomerium

Related issues

• IDP_CLIENT_ID per route for Kubernetes ingress pomerium#5954

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review