[FIX] hr: add private contact to employee's followers

When creating an employee with a private contact,
the contact will not be automatically added to the employee's followers.

The desired behavior is that the contact is added
to the followers even if it has a private address.
Commit: 8760a4d0

However, this behavior will not be possible because
adding a contact with a private address is forbidden.
Commit: 20536e1b

Solution:
The solution is to use the private method `_message_subscribe`.
This is not a problem in the business case.
Indeed, the people who have access to the employee's file
also have access to the private contacts.
Moreover, the access rights of the public method `message_subscribe`
do not concern this business case.

opw-3249646

closes odoo/odoo#118572

Signed-off-by: Kevin Baptiste <kba@odoo.com>