[IMP] mail: split multi-emails partners for outgoing emails

PURPOSE

Be defensive when dealing with email fields, notably when having multi-emails
or email field containing an already-formatted email.

SPECIFICATIONS

When building the final 'email_to' of outgoing emails using 'formataddr' we
have issues if email contains multi emails or formatted email. Main fix of
this commit is to extract emails and rebuild the 'email_to' list based on
found emails.

E.g. partner Raoul - email: "Raoul" <raoul@raoul.fr>
 -> before: to: "Raoul" <"Raoul" <raoul@raoul.fr>> (double format)
 -> after: to: "Raoul" <raoul@raoul.fr>

E.g. partner Raoul - email: raoul1@raoul.fr, raoul2@raoul.fr
 -> before: to: "Raoul" <raoul1@raoul.fr, raoul2@raoul.fr>
    single email with multiple emails, depends on server fault tolerance)
 -> after: to: "Raoul" <raoul1@raoul.fr>, "Raoul" <raoul2@raoul.fr>
    multi emails

Fix that computation by using all normalized emails found in 'email' fields
and rebuilding a formatted email based on name + those emails. We do not
use `email_formatted` as it is not really multi-enabled. We prefer a local
defensive approach to be as tolerant as possible with respect to user inputs.

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#74474