[IMP] hr_contract: add employee type

Prior to this commit:

* All employees were treated the same way regarding the contractual point of vue.

From this commit on:

* Three employee types will be created:
 - Employee, which is supposed to be under contract
 - Student & Contractor which are both not supposed to have an employee contract (but are still use the employee as working for the company)

* Employee view and Contract History will take this consideration into account.

closes odoo/odoo#64931

Taskid: 2446000
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>