Add missing unsubscribe support

[bpotokar]

Currenlty unsubscribing from a topic is not possible. Unity side actually has unsubscribe method, but ROS side does not implement that functionality. As a result, calling unsubscribe causes the AttributeError: 'SysCommands' object has no attribute 'remove_subscriber' exception.

This PR adds the missing method remove_subscriber, which unregisters ROS subscriber so new messages are not sent to Unity anymore.

This bug is reported in this issue on Unity-Robotics-Hub repository.

Here is described the call stack, that causes the exception, in more detail:

If Unity script calls the Unsubscribe function, which calls UnsubscribeAll. This calls SendSubscriberUnregistration, which sends the remove_subscriber command. That results in the following error:

Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "ros_tcp_endpoint/client.py", line 225, in run
    self.tcp_server.handle_syscommand(destination, data)
  File "ros_tcp_endpoint/lib/python3.8/site-packages/ros_tcp_endpoint/server.py", line 114, in handle_syscommand
    function = getattr(self.syscommands, topic[2:])
AttributeError: 'SysCommands' object has no attribute 'remove_subscriber'

[unity-cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Blaž Potokar seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}