add autostart timer option for the Timer class

## Feature request

#### Feature description

`rclpy` counterpart of https://github.com/ros2/rclcpp/pull/2005

see more details for https://github.com/ros2/rclcpp/issues/1980

#### Implementation considerations

`rclpy` implementation also depends on https://github.com/ros2/rcl/pull/1004, with adding `autostart` option flag to `Timer` class.