expanded rpc api to add MempoolSizeChanged

[x100111010]

Confirmed working with the below wrpc example subscriber:
https://github.com/kaspanet/rusty-kaspa/blob/master/rpc/wrpc/examples/subscriber/src/main.rs

self.client().rpc_api().start_notify(listener_id, Scope::MempoolSizeChanged(MempoolSizeChangedScope {})).await?;

Creating listener with options: url=Some("ws://localhost:17210"), network=testnet-10, encoding=Borsh, timeout=5000ms
Received connection event
Connected to Some("ws://localhost:17210")
Node version: 0.17.2
Network: testnet-10
Virtual DAA Score: 106173779
Node is indexing UTXOs: true
Node is synced: synced
Notification: MempoolSizeChanged(MempoolSizeChangedNotification { network_mempool_size: 535 })
Notification: MempoolSizeChanged(MempoolSizeChangedNotification { network_mempool_size: 586 })
Notification: MempoolSizeChanged(MempoolSizeChangedNotification { network_mempool_size: 543 })
Notification: MempoolSizeChanged(MempoolSizeChangedNotification { network_mempool_size: 631 })
Notification: MempoolSizeChanged(MempoolSizeChangedNotification { network_mempool_size: 568 })
Notification: MempoolSizeChanged(MempoolSizeChangedNotification { network_mempool_size: 756 })

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Currently in the kaspa socket server we periodically update mempool size using getInfoRequest. With the rusty-socket-server we can now use wRPC notifications to get mempool size updates directly and efficiently, replacing the full GetInfoResponseMessage with a more lightweight, real-time mempool size notification.