[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/__init__.py

@@ -0,0 +1,2 @@
+from . import timebox
+from . import project_task

project_timebox/models/project_task.py

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+
+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')

project_timebox/models/timebox.py

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