Move the transport connection string api down to the individual transports

[andreasohlund]

As #4274 highlights there is not enough value to have a core api for this. We can just let the transports that needs it expose a .ConnectionString() method.

Msmq wouldn't need this IMO since all the settings shouldn't change that often

https://docs.particular.net/nservicebus/msmq/connection-strings