Added GetMessageSizeAsync to IMailSpool (#1233)
Added GetMessageSizeAsync to IMailSpool (#1233)

* Added GetMessageSizeAsync to IMailSpool

* Update IMailSpool.cs

Co-authored-by: Jeffrey Stedfast <b08229510c3e3ef703ba0cf2ca14382fe3fbf1bd@microsoft.com>
jstedfast/MailKit