[ADD] project_timebox: create addon to estimate task effort

Adds fields to estimate the min and max time needed to solve a task.

Task: [JOI-13](https://justodooit.fr/mail/view?model=project.task&res_id=13&access_token=cca0b860-25d1-472d-9da1-76f68b01a932)

project_timebox/models/timebox.py

@@ -0,0 +1,9 @@
+from odoo import models, fields
+
+
+class Task(models.Model):
+    _name = "timebox"
+    _description = "Timebox"
+
+    name = fields.Char('name', required=True)
+    sequence = fields.Integer()