Usability studies found that static methods are generally not as user-friendly as instance methods. Folks new to the SDK have harder time discovering the APIs, they reverse the flow of typing, and the calling code ends up a bit more verbose because you have to spell out the whole namespace and type name when there aren't any using directives.
There doesn't seem to be a strong benefit or feasibility reason to keep these method statics which are typically harder to use and discover.
cc @kyle-patterson
Part of https://github.com/Azure/azure-sdk-for-cpp/issues/1789
From Jeff:
> BodyStream is only exposed to users when uploading/downloading bytes. Except for Storage almost no services do this at all; they read the stream and deserialize into a structure and the customer gets the structure - not the stream. Azure::Core:IO is a fine namespace.
* Moved `BodyStream` and its derived types from `Azure::Core::Http`
namespace to `Azure::IO`, and update headers.
* Fix up clang formatting.
* Add missing winhttp bodystream namespace change.
* Update merge conflict related changes.
* Update header paths in includes.
