Decouple intent processing from MlsGroup

[insipx]

We currently do much of intent processing in sync, but intents do not necessarily need to be tied to a specific group. Most of the intent processing uses the lower level OpenMlsGroup, rather than the MlsGroup of libxmtp. This means we could create a separation between the abstraction our protocol offers and the lower-level functionality of MLS. Further, Intents mostly use the XmtpOpenMlsProvider, so we can further simplify by either creating a supertrait tied to our provider or by creating implementations directly on XmtpOpenMlsProvider.

In order to simplify intent flow + management, we could pull lots of the intent logic out of sync.rs into its own "subsystem" separate from groups. Group-related intent processing can stay the same

This can also further help decouple MlsGroup from client, which will make it easier to add/customize future features.

Open to having a discussion on this + anything else to improve w.r.t to this. I think @richardhuaaa had some thoughts around this at some point too

came up recently during a small refactor in wasm-backend and I separated one intent processing function from MlsGroup