First pass at adding in transactions

[richardpark-msft]

First pass at adding in transactions without any hacks.

• PerformAttach.Target is an 'any' now, to accomodate that it can be two types (Target/CoordinatorTarget)
• New unmarshaller for Target so we can marshal and unmarshal Target/CoordinatorTarget types properly.
• Added in a few functions to make "peeking" composites a bit easier. Still seems like there's some room to do more here, but what I've got in there seems safe enough.

This also adds in some live testing against Azure Service Bus for transactions.

Functionality all appears to be intact - can send, receive and also discharge transactions (rollback and commit).

[richardpark-msft]

(I have a couple of tests I want to fill in, and I need to make the live tests optional but this is the approach I've got for adding in support for AMQP transactions).

There might be additional work but, AFAICT, this implements the parts of the spec that we care about in a way that could potentially be expanded if needed.