Offline Stateless Testing for Xmtp API

[insipx]

with the new xmtp_api_d14n crate (or even the older MlsTraits, although it may result in more clients) we can create the ability to 'statelessly test' XMTP API functions.

Rather than reach out to a network, we can implement the traits on a struct that holds a simple unbounded channel. The channel can transparently pass data that it got in request to response functions. This way we have much more control over the data that is represented in API calls, allowing us to do complex tests with streaming or updates that would otherwise be very difficult