Allow specifying server version of requests

[leftmostcat]

Versions after Exchange 2007 SP1 added operations, fields, and behaviors which require specification of the RequestServerVersion parameter in the header of the request's SOAP envelope. We need to support these for MarkAsJunk and possibly future additions. At some point, it may be useful to automate minimum version detection based on the fields/operations present in the request, but a first pass is just to provide a public Header struct and an enum with the available Exchange versions which can be provided to the envelope and serialized automatically.