fatxpool: event streams moved to view domain

[michalkucharczyk]

Overview

This pull request refactors the transaction pool graph module by renaming components for better clarity. The EventHandler trait was introduced to enhance flexibility in handling transaction lifecycle events. Changes include renaming graph::Listener to graph::EventDispatcher and moving certain functionalities from graph to view module in order to decouple graph from view-related specifics.

This PR does not introduce changes in the logic.

Notes for Reviewers

All the changes looks dense at first, but in fact following was done:

• The graph::Listener was renamed to graph::EventDispatcher, to better reflect its role in dispatching transaction-related events from ValidatedPool. The EventDispatcher now utilizes the L: EventHandler generic type to handle transaction status events.
• The new EventHandler trait was introduced to handle transaction lifecycle events, improving implementation flexibility and providing clearer role descriptions within the system. Introduction of this trait allowed the removal of View related entities (e.g. streams) from the ValidatedPool's event dispatcher (previously listener).
• The dropped monitoring and aggregated events stream functionalities and related types were moved from graph::listener to the view module. The ViewPoolObserver, which implements EventHandler, now provides the implementation of streams feeding.
• Fields, arguments, and variables previously named listener were renamed to event_dispatcher to align with their purpose and type naming.
• Various structs such as Pool and ValidatedPool were updated to include a generic L: EventHandler across the codebase.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/13272316509
Failed job name: cargo-clippy

[michalkucharczyk]

/cmd prdoc --bump minor --audience node_dev

[iulianbarbu]

LGTM!

[skunert]

Looks good! Just the one comment needs to be addressed.