Super-charging the Postmark.Net client

The Postmark.Net client has been updated to version 2.0, with a range of improvements including complete API coverage, full async/await support, and complete XML documentation. The update aims to provide a more streamlined and efficient client library, with optional parameters reduced and Semantic Versioning adopted. Users are advised that some modifications may be required for projects targeting older versions of .Net, but the 1.6 client will continue to be available in the Nuget package feed.