feat: add fine-grained realm-wide client scope management

[aslafy-z]

Replaces #649 (Closes #649)
Relates #895 #470 #547

This PR adds two new resources to the Terraform Keycloak provider: keycloak_openid_default_client_scope and keycloak_openid_optional_client_scope. These resources allow assigning default or optional client scopes at the realm level, resolving a significant limitation. While default client scopes can already be configured at the realm level, custom-created scopes are not supported due to a chicken-and-egg issue: custom scopes require the realm to exist beforehand. Moreover, these resources enable fine-grained management of individual client scopes rather than managing an entire list, providing enhanced flexibility and precision.

Thanks to @Korsarro69 for the initial work. I opened this PR as #649 is now stale.

cc @daviddelannoy @dglozano @lebronnecf

TODO:

• Naming clarification: Current client scope mapping resources are called keycloak_openid_client_default_scopes and keycloak_openid_client_optional_scopes. Isn't keycloak_openid_default_client_scope and keycloak_openid_optional_client_scope not too confusing? Should I rename to keycloak_openid_client_default_scope and keycloak_openid_client_optional_scope? I guess the real-wide will be added to the list resources in the future, which will align functionalities.
• Fix/Add tests

Run tests in codespaces with:

sudo apt update
sudo apt-get install -y openjdk-11-jdk
make user-federation-example
make local
export KEYCLOAK_CLIENT_ID=terraform \
KEYCLOAK_CLIENT_SECRET=884e0f95-0f42-4a63-9b1f-94274655669e \
KEYCLOAK_CLIENT_TIMEOUT=5 \
KEYCLOAK_REALM=master \
KEYCLOAK_URL="http://localhost:8080"
make testacc TESTARGS='-run "DefaultClientScope|OptionalClientScope"'

[denniskniep]

@aslafy-z are you still working on that PR? Do you have any plans when to continue or complete that?

[aslafy-z]

For some reason, TestAccKeycloakOpenidClientDefaultScopes_authoritativeRemove becomes flacky with this addition. I guess it's linked to these new added tests that interacts with the same Realm as TestAccKeycloakOpenidClientDefaultScopes. Any idea why and how this may be resolved @denniskniep?