`fatxpool`: event streams moved to view domain (#7545)

#### 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`](https://github.com/paritytech/polkadot-sdk/blob/515cb404/substrate/client/transaction-pool/src/graph/listener.rs#L74C12-L74C27),
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`](https://github.com/paritytech/polkadot-sdk/blob/515cb404/substrate/client/transaction-pool/src/graph/listener.rs#L34)
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](https://github.com/paritytech/polkadot-sdk/blob/515cb404/substrate/client/transaction-pool/src/fork_aware_txpool/view.rs#L157-L188)
and [related
types](https://github.com/paritytech/polkadot-sdk/blob/515cb404/substrate/client/transaction-pool/src/fork_aware_txpool/view.rs#L112-L121)
were moved from `graph::listener` to the `view` module. The
[`ViewPoolObserver`](https://github.com/paritytech/polkadot-sdk/blob/515cb404

/substrate/client/transaction-pool/src/fork_aware_txpool/view.rs#L128C19-L128C35),
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.

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Iulian Barbu <14218860+iulianbarbu@users.noreply.github.com>