Make group approvals conditional based on config

[jfreda]

This PR makes group approvals conditional on a new config option.

Breaking changes - config.hcl

Previously (from #660):

google_workspace {
  ...
  // groups_prefix is the prefix to use when searching for Google Groups.
  groups_prefix = "team-"
  ...
}

Now:

google_workspace {
  ...
  // group_approvals is the configuration for using Google Groups as document
  // approvers.
  group_approvals {
    // enabled enables using Google Groups as document approvers.
    enabled = true

    // search_prefix is the prefix to use when searching for Google Groups.
    search_prefix = "team-"
  }
  ...
}