sync: Send already connected peers to new subscribers (#7011)
Introduce `SyncEvent::InitialPeers` message sent to new subscribers to allow them correctly tracking sync peers. This resolves a race condition described in https://github.com/paritytech/polkadot-sdk/issues/6573#issuecomment-2563091343. Fixes https://github.com/paritytech/polkadot-sdk/issues/6573. --------- Co-authored-by: command-bot <> Co-authored-by:Bastian Köcher <[email protected]> (cherry picked from commit b4177a9f)