Added config for OAuth login/logout URIs

[plnordquist]

Description of the change

These changes allow overriding the OAuth Login and Logout URIs which is useful when deploying the chart under a sub path. For my deployment, I set the ingress to rewrite urls and change a few other values to properly know about the sub path. I've included some sample values below however the sample values would need extra configuration to fully configure the authProxy service.

• Relevant Sample Values:

authProxy:
  enabled: true
  additionalFlags:
  - -redirect-url=https://<hostname>/kubeapps/oauth2/callback
  - -cookie-path=/kubeapps
  oauthLoginURI: /kubeapps/oauth2/start?rd=/kubeapps/
  oauthLogoutURI: /kubeapps/oauth2/sign_out
ingress:
  enabled: true
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /$2
  extraHosts:
  - name: <hostname>
    path: /kubeapps(/|$)(.*)
  extraTls:
  - hosts:
    - <hostname>
    secretName: <hostname>-cert

Benefits

• Allow configuring the locations that Kubeapps should redirect a user to when logging in or out with authProxy.enabled=true

Possible drawbacks

Applicable issues

• Partially related to Cannot use external auth proxy service without patching config #1579, would need more changes to allow an external auth proxy service

Additional information

[absoludity]

Excellent, thanks @plnordquist . That improves things either way.