Allow Described types in SimpleValue to enable interoperability with .Net EventHubs client

[LarryOsterman]

Fixes #311

[minghuaw]

LGTM!

[LarryOsterman]

LGTM!

TYVM. And yeah, this one was really surprising to me when the SDK for Python folks showed me the trace. And then to discover that there's a set of magic elements that aren't documented which get converted differently was also a surprise.

FWIW, the EventHubs client for Go won't do the conversions - they treat them as a private enumeration and nobody has mentioned that it's a problem (OTOH, the Python team ran into an issue where someone added a DateTimeOffset in .Net, it was converted to a described type , but when Python tried to map it back to the Python DateTimeOffset type, the Python version couldn't handle the representation of the .Net value (there's a similar problem in Rust EventHubs with the -62_135_596_800_000 value - that was fun because it worked fine on some Linux devices but failed miserably on Windows)