Streaming bindings

[neekolas]

Summary

• Builds on top of Allow streaming group messages #397 and Add support for conversation streaming #398
• Adds bindings for message streaming

Message streaming requires the caller to implement a class in Swift/Kotlin that meets the FfiMessageCallback interface defined in the UDL file. The on_message function will be called once per message.

Notes

I tried very hard to figure out a way to wrap a Stream type into something FFI friendly. The main issue is that our group.stream() method keeps a reference to self for the life of the stream, which in turn has a lifetimed reference to Client. This is required so that we can actually decrypt messages. The problem is that Uniffi doesn't play nicely with lifetimed references. We can't store a MlsGroup on any sort of FFI friendly struct, and so the reference always gets dropped before the stream is done. This makes the compiler unhappy.

Maybe someone smarter than me can figure it out (I've tried every permutation of boxes/arcs/mutexes I can think of), but I have to admit the callback interface might actually be easier in the end. At least we aren't exposing any sort of exotic types to Swift/Kotlin. The memory management should be simpler since the stream never leaves Rust-land.

[nplasterer]

LGTM. 👍 I like the evolution of Intro, Invite, Welcome. 😄