[12.0] new addon - enable sending email on certain request stage changes

[dalonsod]

We'd like an addon with the following features:

• when a maitenance request is created, send an email to a set of contacts defined as a general parameter.
• when a maintenance request is closed, send an email to a set of contacts depending on the maintenance equipment.

To achieve this,

• General parameter can be placed making this addon depending on https://github.com/OCA/maintenance/tree/12.0/base_maintenance_config.
• Sending emails is a common feature in Odoo, we can copy code from e.g. OCA Helpdesk tickets, that uses Odoo tools and templates for email compose and send.

cc @lmiguens-solvos @ChristianSantamaria