[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/project_task.py

@@ -0,0 +1,9 @@
+
+from odoo import models, fields
+
+
+class Task(models.Model):
+    _inherit = "project.task"
+
+    timebox_min_id = fields.Many2one('timebox', string='Timebox Min')
+    timebox_max_id = fields.Many2one('timebox', string='Timebox Max')