sso_proxy: reduce amount of group validations

[Jusshersmith]

Problem

With the validator abstraction work that was recently done we inadvertently started to run group validations for each authenticated request. See 'Notes' section for specific details.

This increased volume of requests increases the potential to cause extra strain on upstream providers

Solution

We don't need to validate the groups again here. This pull request brings us closer to previous functionality where we re-validate group membership after refreshing or validating the session, and re-validate email domains and addresses upon each request.

Notes

• Now that the group membership check is an official 'validator' within sso-proxy it's ran each time we call RunValidators().

  The problematic call in question:

  sso/internal/proxy/oauthproxy.go

  Line 784 in 9019d4f

  errors := options.RunValidators(p.Validators, session)

  Previously, we were only checking email address/domains here, with the group check being ran just above that when refreshing or validating the session:

  sso/internal/proxy/oauthproxy.go

  Line 731 in 9019d4f

  ok, err := p.provider.RefreshSession(session, allowedGroups)

  &

  sso/internal/proxy/oauthproxy.go

  Line 762 in 9019d4f

  ok := p.provider.ValidateSessionState(session, allowedGroups)

• An alternative solution to validator pkg: control when each validator is ran #266

• Also included in this is a change to the group validator (it's no longer used as a pointer). Largely to bring it in line with the other validators.