perf: Don't handle expensive requests for rooms we aren't in

Mostly borrowed from dendrite:

https://github.com/element-hq/dendrite/blob/a042861/federationapi/routing/routing.go#L601

src/api/server/send_join.rs

@@ -4,7 +4,7 @@ use std::{borrow::Borrow, time::Instant, vec};
 
 use axum::extract::State;
 use conduwuit::{
-	Err, Event, Result, at, debug, err, info,
+	Err, Event, Result, at, debug, debug_warn, err, info,
 	matrix::event::gen_event_id_canonical_json,
 	trace,
 	utils::stream::{BroadbandExt, IterStream, TryBroadbandExt},
@@ -36,6 +36,18 @@ async fn create_join_event(
 	if !services.rooms.metadata.exists(room_id).await {
 		return Err!(Request(NotFound("Room is unknown to this server.")));
 	}
+	if !services
+		.rooms
+		.state_cache
+		.server_in_room(services.globals.server_name(), room_id)
+		.await
+	{
+		debug_warn!(
+			origin = origin.as_str(),
+			"Refusing to serve send_join for room we aren't participating in"
+		);
+		return Err!(Request(NotFound("This server is not participating in that room.")));
+	}
 
 	// ACL check origin server
 	services