[FIX] mail: fetches the attachments checksum for cache control.

This commit makes so that we also fetch the checksum of the attachments
of mail.messages to use it for cache control.

Before this commit, the urls of message attachments had
`undefined` for the signature parameter.

addons/mail/models/mail_message.py

@@ -977,6 +977,7 @@ class Message(models.Model):
             attachment_ids = []
             for attachment in message_sudo.attachment_ids:
                 attachment_ids.append({
+                    'checksum': attachment.checksum,
                     'id': attachment.id,
                     'filename': attachment.name,
                     'name': attachment.name,