Add support for adding attachments as LinkedResources in MailMessage

[johnwc]

Describe the solution you'd like
We need to be able to add attachments that are considered LinkedResources to the MailMessage object, so the attached/embedded images can be used within the html with their ContentId. I will create a PR for this update.

@jamesmh Within this PR, is it ok if I add a new property to the Attachment class that allows using a Stream? So that a developer can pass a byte array or a Stream.

[johnwc]

@jamesmh thoughts?