Provide more ways for middleware to interact with mail parts

[wneessen]

This PR adds functionalities for middlware (in particular the new OpenPGP middleware) to provide more access to the Msg.

In detail:

• The File type now has additional fields: ContentType, Desc and Enc with corresponding `FileOption´ methods to stear those
• A new PGPType has been added which can be set as part of the Msg struct using the WithPGPType option method and a SetPGPType setter method
  • 3 PGPTypes have been added: NoPGP, PGPEncrypt and PGPSignature
  • If a PGPType is set, the msgWriter will produce different MIME headers for the content types
• Msg parts received a description field which will be reflected in the Content-Description header of the part. Corresponding Option-, getter- and setter methods have been added: WithPartContentDescription, GetDescription, SetDescription