Display message/rfc822 mime parts

Is your feature request related to a problem? Please describe.

I'd like to suggest that the Mail app should display message/rfc822 mime parts.

Those parts often are forwarded emails, or emails that were encapsulated by mailing list managers to produce valid DMARC headers. More often than not I (and I guess many others) want to actually read those messages, without having to download and open them in another app.

Displaying the message/rfc822 mime part should not replace other content parts of the email but it should be displayed additionally.

Describe the solution you'd like

The Mail app should show an rendering of the mime part that includes the typical headers (From, To, Subject, etc.) and displayable content parts just as it would if the mime part would be a standalone email.

Bonus points for allowing to extend the list of headers, but I would not consider that a necessary feature.

Start at the Mail app's existing rendering path for standalone messages and trace how MIME parts are selected. Compare how a message/rfc822 part should present typical headers and displayable content with a standalone email. Done means the nested message is readable, while the parent email's other content parts remain visible.