Feature MimeMessage testable via an interface

[rogatec]

Is your feature request related to a problem? Please describe.
As already discussed in the closed issue #835 - I am currently running in a rare scenario where the MessageId is null.
To test my code, I need to create a wrapper class with an interface to mock it in my tests.

Describe the solution you'd like
An interface e.g. IMessage or iMimeMessage where MimeMessage relates to.

Describe alternatives you've considered
As I am writing no alternative comes to my mind 😞

Additional context
For additional context please see #835 .

[jstedfast]

I'm not sure if you use MailKit or not, but all of the (core) MailKit classes have corresponding interfaces.

I've been meaning to do this for a while now with MimeKit as well, but lacked "reason" to do it.

Now I have an excuse to do it :-)