Document design for transport::UTransport

[gregmedd]

The base UTransport object is, in theory, pretty simple. The bulk of the work will be in the individual transport implementations (e.g. ZenohUTransport). However, it is possible that UTransport might need to do a little more than just pass calls through to its virtual interfaces. This design should consider:

• If UTransport needs a thread pool for callbacks (so the transport implementations don't have to do it)
• If UTransport should do anything to prevent multiple parallel calls to the same callback (so callbacks don't have to add extra thread safety concerns)
• If there are any threads, how do they get shut down?
• (possibly other things?)

[cbeck83]

Recommend Uzi for this one