fix(user): ignore special groups (BUILT_IN and APP_GROUP) in the group_memberships field

[rasta-rocket]

Currently it is not possible to manipulate the group memberships of a user if it is belonging to special groups such as "APP_GROUP" (group.type).

Error: [ERROR] Error Updating Groups On User: The API returned an error: You do not have permission to perform the requested action

  on user-bfernando.tf line 3, in resource "okta_user" "test":
   3: resource "okta_user" "test" {

According to the debugging that I did, this appears when the provider tries to remove the set of groups assigned to the user:
https://github.com/terraform-providers/terraform-provider-okta/blob/master/okta/user.go#L470

However assignments related to special groups (BUILT_IN and APP_GROUPS) cannot be manipulated through the Okta API (even as SUPER_ADMIN)

Note: documentation about group types

Today this situation is a kind of already managed by skipping the built-in group called Everyone:
https://github.com/terraform-providers/terraform-provider-okta/blob/master/okta/user.go#L469

This skip phase should be generalized to groups such as BUILT_IN and APP_GROUP group types.

[bogdanprodan-okta]

Hi @rasta-rocket! I will make an additional investigation on this issue and merge this PR if everything is OK. It then should appear in the new release v3.7.5 in early 2021.