related #96

Continuing on from #99, this PR creates a permanent fix to background requests accumulating CSRF cookies upon auth token expiry. Specifically, this PR allows enabling the oauth2-proxy sign-in page, which will require a user to click "sign in" before they are redirected to dex (which triggers a CSRF cookie to be added).

When enabled, this will prevent all but the most determined users from exceeding the header limits (and getting 431 errors), because the only way to cause a CSRF cookie to be added is to manually start a dex auth flow.

However, because this change will break our existing KFPClientManager() class, we will leave the sign-in page disabled by default in deployKF 0.1.X, and only enable it by default in 0.2.0.

NOTE: cluster admins who are seeing lots of 431 errors can enable it before then by setting the deploykf_core.deploykf_auth.oauth2Proxy.enableSignInPage value to true:

deploykf_core:
  deploykf_auth:
    oauth2Proxy:
      enableSignInPage: true