Support Passing Back Gmail Message and Thread Ids

[jpulec]

It's pretty common for an application to want to know the message id and thread id that gmail has assigned to a sent message. Currently, this backend doesn't provide any way to access the Gmail response data.

Is this something you would consider supporting for this project?

One way it might be possible would be to support passing a special EmailMessage subclass say something like GmailMessage and if the backend detects this subclass is used, it could assign id and threadId to attributes on it.

Happy to help with a PR if this sounds like something that this library should handle!