feat(driver): add notification for the beginning of trying to settle the solution

[kirsanium]

Problem

I am a developer in Horadrim solver team.
Some PMMs benefit from knowing that their quotes will be used in settlement as early as possible because they can hedge earlier, manage their risks better, and as a result provide better prices. The example would be #3452 feature request from Liquorice PMM - they solved their problem in PR #3451 by implementing a notification system that can notify PMM directly from driver. However, this approach requires PMMs to be involved in pushing a PR to cowprotocol, which is not always possible/feasible/desirable.

Suggested solution

I suggest the possibility to lay responsibility for notifying PMMs on solvers' shoulders by implementing a new notification type for solvers, that would fire off whenever a solver has won and the settlement is about to begin.

Implementation suggested in #4327.

Alternatives considered

None.

Additional context

This feature would also make the solvers relying on reference driver more competitive comparing to the ones with their own driver.

Acceptance criteria

• Implement a new notification type in driver and make that fire off before the settlement begins.