Changes to SOAP structure to support optional Header

[tobypilling]

Hey gents, I started looking into Mark as Junk, and realized that we'll likely need to support Soap:Header for some operations that are only available on specific versions of EWS. Here's an example:

https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/markasjunk-operation#markasjunk-operation-request-example-add-a-sender-to-the-blocked-sender-list

[leftmostcat]

Thank you for identifying the need for versioning for the MarkAsJunk operation. That said, this isn't the right approach for SOAP headers. There's not really any need for them to be generic as far as I can tell, and existing serialization machinery can be used instead of hand-written.

Since you won't be doing development long-term, my preference is to either have Brendan or me tackle this or to have another dev start from discussing design and implementation. I've cut #32 for the work either way.