Make `IMailFolder.GetMessage()` return `IMimeMessage`

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

I'm currently trying to write unit tests for my services. One of these services uses MailKit to fetch some messages with attachments.

I can almost mock everything (using IImapClient, IMailFolder) except MimeMessage, because the Attachments property is not overridable.

Describe the solution you'd like

I'd like IMailFolder to return IMimeMessage on GetMessage/GetMessageAsync, so that I can return a mock implementation.

Describe alternatives you've considered

I considered simply creating a non-mocked instance of MimeMessage, which works fine. Still, it would be nice to be able to properly mock every aspect (for call counting, exception throwing, etc.)