graph: Allow to disable the default role assignment on user creation

Introduces a switch ('GRAPH_ASSIGN_DEFAULT_USER_ROLE') to allow to disable the assignment of the default role "User" to newly created users.
This will be used for setups where the role-assignments are populated either manually or during first login (e.g. from OIDC claims)

services/graph/pkg/service/v0/users.go

@@ -328,7 +328,7 @@ func (g Graph) PostUser(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// assign roles if possible
-	if g.roleService != nil {
+	if g.roleService != nil && g.config.API.AssignDefaultUserRole {
 		// All users get the user role by default currently.
 		// to all new users for now, as create Account request does not have any role field
 		if _, err = g.roleService.AssignRoleToUser(r.Context(), &settings.AssignRoleToUserRequest{