Getting the MIME content of a message from office.js

Please add a way to get the MIME content of an e-mail message in office.js

We are producing an Outlook Addin and need to resort to Outlook REST API to get an e-mail message in MIME (.eml) format.

Unfortunately Outlook REST API is deprecated. The alternative, MS Graph doesn't work with on prem Exchange.

So please add support for getting MIME content in office.js so we don't need to additionally use a web API.