[ADD] {google,fetchmail}_gmail: OAuth for gmail servers

Purpose
=======
Less secured apps are no longer supported by google, therefore, we need
to transition to the OAuth2 authentication system.

Specifications
==============
1. User will need to fill their Gmail API credentials in the main
   settings page
2. Then, in the incoming / outgoing mail server form view, they will
   need to tick the Gmail support checkbox
3. A link will be available to be redirected to Gmail and accept the
   permission
4. The user can now copy / paste the authorization code in Odoo, set
   his email as "login" and then send / receive emails with Gmail

Task-2170676

closes odoo/odoo#83388

X-original-commit: 887524b7
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>