Allow passing context to subscription callbacks

[BrettRD]

Following from #35 add context pointers to service calls, I'd like to use subscriptions with context pointers too

The change would re-use (and re-name) the void * service_context; added in #45, and add the following:

typedef void (* rclc_subscription_callback_with_context_t)(const void *, void *);

rcl_ret_t
rclc_executor_add_subscription_with_context(
  rclc_executor_t * executor,
  rcl_subscription_t * subscription,
  void * msg,
  rclc_callback_t callback,
  void * context,
  rclc_executor_handle_invocation_t invocation);

I have most of a patch, I'll re-base it onto #45 in the next couple of weeks and submit a pull to micro-ros

[JanStaschulat]

Just for information: in future create a feature branch from ros2/rclc and create a pull request directly to ros2/rclc.
See also ticket micro-ROS#75

[BrettRD]

implemented in #107, but needs #106 first
sorry about the pull to foxy, that's the only branch I'm set up to test on.

[JanStaschulat]

@BrettRD, just let me know, when you are done, then I'll do a code review.

[BrettRD]

I think it's ready, I was expecting way more merge conflicts