feat: Add keycloak_groups_permissions for admin_fine_grained_authz

[hoeggi]

This is a to add support for fine grained permissions on groups, it's somewhat similar to #409

I needed this internally so it is still missing documentation and tests. I marked this PR as WIP to ask if you are interested in this and if yes i will finish up with documentation and add the test cases.

Edit: I already added some documentation and a basic test case. Let me know if you would like anything more.

[mrparkers]

This looks great, thanks for the PR! The only thing I'd like to be changed is to rename this resource from keycloak_groups_permissions to keycloak_group_permissions, as this resource operates on a single group.

You may have borrowed this naming convention from keycloak_users_permissions, but that resource deals with permissions for all users in a realm (via /realms/${realmId}/users-permissions), whereas this resource is only looking at a single group.

Other than this change, everything else LGTM.

[hoeggi]

Yes you are right, i did borrow lot's of existing code :)

I did a full rename from groups to group on all filenames/resources.

[mrparkers]

Sorry for the late approval. Everything looks great, thanks for the PR!