feat(sdk): SlidingSyncRoom is cheap to clone.

This patch changes `SlidingSyncRoom` to move all its fields inside an
inner type `SlidingSyncRoom` behind an `Arc`, so that cloning is cheap,
and all clones are sharing the same state.

Author: Hywan

crates/matrix-sdk/src/sliding_sync/list/mod.rs

@@ -28,7 +28,7 @@ use tracing::{instrument, warn};
 use super::{Error, SlidingSyncInternalMessage};
 use crate::Result;
 
-/// Holding a specific filtered list within the concept of sliding sync.
+/// Holding a specific filtered list within the concept of Sliding Sync.
 ///
 /// It is OK to clone this type as much as you need: cloning it is cheap.
 #[derive(Clone, Debug)]