use `client_credentials` grant type

[belfhi]

If "service accounts roles" is activated in the Keycloak OIDC Client Settings, and adds the appropriate roles to be able to query-users and query-groups, one can use the grant_type=client_credentials in the request and does not need to add an extra user with the realm roles.

This would simplify configuration since only client_id and client_secret needs to be supplied. More documentation on how the client needs to be set up should be added, though.

[tomako]

Thank you @belfhi , I will test this and update the documentation soon.