This commit adds methods to easily obtain calendar validity per resource
for a given search period and get the work intervals per resource,
taking into account the validity of their calendars.
In hr, the create and departure date of an employee are taken into
account to compute the calendars validity.
In hr_contract, it looks for every contract of human resources with
employee of type student or employee existing in the search period and
consider the specific calendar of the contract as valid during the
contract-lifetime.
PR: #77362
task-2646630
Purpose
=======
An employee and its current contract should have the same
calendar. Currently, if the contract's calendar is changed,
the employee's calendar is not changed.
In the same way, if a contract is created directly in `open`
state, the employee's calendar is not changed.
Finally, the calendar synchronization when the employee
contract is changed is only done in payroll, for no reason.
Specification
=============
Fix the three issues described above.
If the employee's calendar is nonetheless manually changed, a warning
should be displayed if there is a mismatch between both calendars.
Thibault Libioulle