[FIX] hr_timesheet: Fetch server time before render

1) Add a hidden optional field to the timesheet tree view.
2) Try to display that field by checking the checkbox.
=> the column appears then directly disappears.

Asynchronous calls (here: fetching the server time) should
always be in the `willStart` method.

Task 2204084

closes odoo/odoo#48775

X-original-commit: 39fa08c4d6a1b467b520bdd0f75950d2c33f4a2e
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>
Signed-off-by: lul-odoo <LucasLefevre@users.noreply.github.com>