feat: add latest_barrier method to CheckpointExt trait

[alessandromazza98]

This PR adds the latest_barrier method to the CheckpointExt trait with a default implementation.

This is an helper method that returns the latest barrier checkpoint in the history of this checkpoint, if any. It returns None if no barrier checkpoint is found in the history.

[julio4]

Curious to know in which case you are using this method?
history_staging could be related

[alessandromazza98]

Curious to know in which case you are using this method? history_staging could be related

this method is definitely not 100% needed, meaning that I can already obtain that data without this helper method, but it's very helpful so that I don't have to perform this operation manually when I need it.

The history_staging is similar but it returns a Span including all history from latest barrier. But latest_barrier returns the specific barrier Checkpoint. This is useful when I know that I saved something in the context of the barrier checkpoint and I want to grab it.

an example, inside the build_payload fn of the Platform trait:

if let Some(latest_barrier) = payload.latest_barrier() {
	// clone is cheap because it's mostly Arc
	if let Some(built_payload) =
		latest_barrier.context().maybe_built_payload.clone()
	{
		return Ok(built_payload);
	}
}
// ... rest of the code