Add an optional API to StateMachine.DataStream for providing an Executor

Currently, DataStreamManagement has a writeExecutor for executing the streaming tasks. In this JIRA, we propose to add an optional API to StateMachine.DataStream so that a StateMachine can choose to provide an Executor. Then the StateMachine can have more control on the thread management.

The new API may look like below.

    /**
     * @return an {@link Executor} for executing the streaming tasks of this stream.
     *         If the returned value is null, the default {@link Executor} will be used.
     */
    default Executor getExecutor() {
      return null;
    }