Add generic BlockStateNotificationStream type for pending, safe, finalized watchers

[jenpaff]

Closes #10204

Usage Examples:

let pending_state = BlockState::new(pending_block);
let (_, rx) = watch::channel(Some(pending_state));
let mut stream = BlockStateNotificationStream::<BlockState>::new(rx);

let finalized_state = SealedHeader::new(Header::default(), hash);
let (_, rx) = watch::channel(Some(finalized_state));
let mut stream = BlockStateNotificationStream::<SealedHeader>::new(rx);

[mattsse]

clean

this will be useful combined with #10231