Add envelope information to mail.Headers

[robdefeo]

Is your feature request related to a problem? Please describe.
From a mail message object there is not easy way to find out what envelope type was used to send it.

Describe the solution you'd like

Add envelope information to mail.Headers.

• add header to rfc2822.EncodeNewMessage and set value e.g. X-Mailchain-envelope-type: 0x01
• add header to rfc2822.EncodeNewMessage and set value e.g. X-Mailchain-envelope-type-encoding: encoding=hex/0x-prefix
• use encoding.TypeHex0XPrefix to create encoding=hex/0x-prefix portion
• expand mail.Headers to include EnvelopeType and EnvelopeTypeEncoding
• expand rfc2822.parseHeaders to read envelope information`

[catacioba]

I would like to work on this one

[stale]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Mailchain team are limited, and so we are asking for your help. If this is a bug and you can still reproduce this error on the master branch, please reply in order to keep the issue open. If this is a feature request, and you feel that it is still relevant and valuable, please reply in order to keep the issue open. This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.