[FIX] mail: bad CTE/QP decoding of rfc822-headers

Based on RFC3462, a Content-Type text/rfc822-headers
exists and provide a mechanism to label and return
only the RFC 822 headers of a failed message (bounce)

These are only the headers and not the full message.

The Content-Type-Encoding should be either 7-bit(US-ASCII)
or Quoted-Printable (QP) as in the section 2 of the RFC.

Spawn the error:
After getting reported by a customer, I had to reproduce
by spamming wrong outlook addresses and add logging
in a sh database on the message_process of mail_thread.py
and logged the `message` variable.

After few retry, I got one of the part that was defined
as followed:
Content-Description: Undelivered Message Headers
Content-Type: text/rfc822-headers
Content-Transfer-Encoding: quoted-printable

The `get_payload()`function used was only assuming
that there is a full email on that part and that
it could only be encoded as an email, which was not
the case in this situation (quoted-printable:
76 characters per line, character `=` used
as the end of line character).

opw-3064589
task-3131561

closes odoo/odoo#109770

Signed-off-by: Julien Castiaux <juc@odoo.com>
Co-authored-by: Baptiste Vergote <bve@odoo.com>