[18.0][ADD]fleet_vehicle_assignation_log_datetime: add new module Replaces #184

[rpsjr]

Replaces #184

Motivation
The standard driver assignation log in Odoo tracks assignments by date, which lacks the precision needed to determine responsibility for time-sensitive events like traffic infractions. This module introduces datetime precision to solve this problem.

Implementation
Adds datetime_start and datetime_end fields to fleet.vehicle.assignation.log.
Replaces the original date fields in the view with the new datetime fields for a better user experience.
Keeps the original date fields invisible and synchronized via onchange methods to ensure backward compatibility.
Overrides fleet.vehicle.write() to set the datetime_start and datetime_end to the exact moment a driver is changed from the vehicle's form view.