Implement session streaming

[onkoe]

To get a game loop going, we'll need to define a 'session' - which is just a fancy term for "singleplayer or multiplayer" save streaming.

In my view, a shared::Session type would be preferable, as both the server and client rely on differentiating the two kinds of sessions.

However, I haven't chosen how I want a session's connection to look. We'll see.

IPC

Previously, I intended to use local WebSockets for local session streaming. After reading into Eclipse Zenoh, though, I think Eclipse iceoryx2 is my preference!

Before, I was thinking that I would likely write a server that mostly focuses on multiplayer, and simply permits singleplayer sessions with local WebSockets. iceoryx2 changes things, though: it may be worth focusing on singleplayer first.

It's also worth noting that iceoryx2 [doesn't currently support iOS/Android](https://github.com/eclipse-iceoryx/iceoryx2, so they may need an IPC-less fallback. To be honest, though, even Bevy has trouble supporting mobile devices...

WAN

For long-distance (i.e., server) connections, I'm not quite sure what I want yet. Here's a list of minor complaints:

• WebSockets (tungstenite): these are common but use TCP underneath, so they're slooooow
• gRPC (tonic): uses HTTP/2 :p
• WebTransport (wtransport): kinda uncommon and based on a draft specification
  • This might be nice if I'm careful with versioning and expectations... 😭

Even with the benefits of WebTransport, it probably won't work right in the browser yet, and I'm still left building a whole web server. Seriously, I might throw something together using a server framework like axum and call it a day. These things are giving me a headache...