Clock publisher works in dedeciated thread

[szylis]

Clock publisher that works in a dedicated thread and it is time scale independent.

Time scale independent means:

• clock always publishes at the required frequency regardless of the the timescale value,
• time difference in messages timestamp is impacted by the time scale value.

The PR includes:

• clock publisher that start new Clock Thread and use this thread to publish time messages,
• time source DotNetSystemTimeSource.cs - thread-safe class for reading the .NET system time,
• TimeScaleProvider - class that provides and synchronizes timescale value between clock and main threads.

This PR also adds a TimeSourceSelector object to the main AWSIM scene, in order to select correct TimeSource to be used by the SimulatorROS2Node

[mackierx111]

LGTM