Camel mail multipart messages do not include all body parts

Camel mail does not include all MIME body parts in multipart messages. In the unit test linked below, the `htmlPart` body part is not contained in the body parts of the deserialized message. This test can easily be broken by simply changing the order that the MIME body parts are added to the multipart message in the `populateMimeMessageBody` method.

https://github.com/apache/camel/blob/main/components/camel-mail/src/test/java/org/apache/camel/component/mail/MimeMessageConsumeTest.java

Users expect to be able to send multipart emails which contain both plain text and html parts (multipart/alternative) so that the users email clients can display it appropriately. This is not possible with Camel, since only one of the two alternative body parts is deserialized.