code: public Observable<RoleDefinitionInner> getByIdAsync(String roleDefinitionId) {
          return getByIdWithServiceResponseAsync(roleDefinitionId).map(new Func1<ServiceResponse<RoleDefinitionInner>, RoleDefinitionInner>() {
              @Override
              public RoleDefinitionInner call(ServiceResponse<RoleDefinitionInner> response) {
                  return response.body();
              }
          });
      }
docs: Gets a role definition by ID.
  
  @param roleDefinitionId The fully qualified role definition ID. Use the format, /subscriptions/{guid}/providers/Microsoft.Authorization/roleDefinitions/{roleDefinitionId} for subscription level role definitions, or /providers/Microsoft.Authorization/roleDefinitions/{roleDefinitionId} for tenant level role definitions.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the RoleDefinitionInner object