Set MID field on MailPiece

[mdconatser]

As a developer, I want the MID field of MailPiece to be set based on data from the original image metadata in the USPS email.

Acceptance Criteria

• MailPiece.MID field is set based on the mail image from the original email's metadata
• Document any relevant content in the Programmer guide (draft thoughts of usage and importantly the "Why")
• Updates SRS and STP (and other docs if necessary)
• Add/Update relevant unit tests

[mdconatser]

Asking customer if this is the "Content-ID" of the image or some substring of that, because I'm not seeing any MID in the header data.