Expose MockTransport for use in other repositories

Hello,

I've been working on up-streamer-rust, and in the course of writing some tests, I found the need to mock some uTransports to test error conditions. I saw MockTransport in https://github.com/eclipse-uprotocol/up-rust/blob/main/src/utransport.rs and think it would be very useful to have in that repo, so am proposing to make that a pub. If that's acceptable, I will make the PR for it.