Add Stream type for the pending block

[mattsse]

Describe the feature

As prep for #5617

we need a helper stream type that makes use of the watch channel introduced in #10203

this should be a new type that wraps the receiver and emits new pending values, None values can be skipped.

see also:

reth/crates/chain-state/src/notifications.rs

Lines 38 to 44 in e907f0e

	/// A Stream of [`CanonStateNotification`].
	#[derive(Debug)]
	#[pin_project::pin_project]
	pub struct CanonStateNotificationStream {
	#[pin]
	st: BroadcastStream<CanonStateNotification>,
	}

see also tokio_stream::WatchStream

Ideally this stream type is generic over Option<T> so we can reuse this for the safe and finalized watchers as well

Additional context

No response