Send skin plugin message after session spawn

[auvq]

Fixes #6288.

When Floodgate's websocket is the only subscriber on the skin API, Geyser itself delivers the skin to the backend via a plugin message. On Velocity this can land in the null-connection window right after player auth but before the backend connection is ready, and gets silently dropped. Player ends up as Steve/Alex to Java players.

Fix: wait until the session is actually spawned before sending. isSpawned flips to true when the Java server sends the player's first position packet, which only happens after the proxy-backend transition is complete. Polls every 500ms with a 5s safety cap.

Haven't tested live, the timing window is hard to reliably hit.

[Camotoy]

Thanks for the PR. I don't think this is the most ideal way to solve the issue - as much as Yet Another Geyser Session Variable is amazing (sarcasm), I think I would rather save this data to a list or variable and check/send that when spawning actually happens.

[onebeastchris]

I agree with Camotoy - a session variable (or maybe on SessionPlayerEntity?) would be nicer compared to continuously re-polling. Happy to merge after that's implemented :)