Put some extensions into their own namespaces

For instance, putting `XR_EXT_debug_utils` definitions into `xr::ext::DebugUtils` would mean that a C++ client would be able to write `xr::ext::debugUtils::Messenger` instead of `xr::DebugUtilsMessengerEXT`. Since setup and teardown will likely be using many related classes, this allows clients to declare a scoped `using namespace` statement and produce significantly less verbose code.

No files, tests, or entry points are named. Start by locating the extension definitions and related setup and teardown classes in OpenXR-Hpp, then assess the proposed namespace structure and its effect on existing C++ usage. Done means the namespace organization and compatibility expectations are specified clearly enough to implement and verify.

Written by the indexing model from the issue text.