Expose the queue_size functionality in the RegisterSubscriber sys_command

[StephanHasler]

Is your feature request related to a problem? Please describe.
In some rooms we have a weak Wifi connection between Unity and ROS. When receiving data from ROS this can have the effect, that we receive very old data, as the RosSubscriber has an infinite buffer.
Because of this, I would like to be able to set the queue_size parameter when calling the RegisterSubscriber sys_command.
This is similar to #82, where this feature was requested for the publisher.
So e.g. I could set the queue_size=1 for data where I'm just interested in in last message. This would save a lot of bandwidth potentially.

Describe the solution you'd like
I guess the solution is the same as for #82, we just need a way to pipe the queue_size parameter from unity to SysCommand/RosSubscriber/rospy.Subscriber

[sarah-gibson]

@StephanHasler , thanks for reporting this. We've made an internal ticket for this [AIRO-1942] and added it to our backlog.