Enable sending an email to users not found in the Google Workspace directory

[jfreda]

We have encountered rare cases where users may not be found in the Google Workspace directory, which prevents them from logging into Hermes. This PR enables sending an email to these users with custom remediation instructions.

New config block

google_workspace {
  ...
  user_not_found_email {
    body    = "Oh no! :-("
    enabled = true
    subject = "Unable to sign into Hermes"

  }
  ...
}