Reflection service does not consult ExtensionRegistry

It just notices extensions that are pulled in from the services themselves. That'll miss most extensions.

We'll probably need to add a getter to `ProtoLiteUtils`.

This may be a good time to add an extra getter/setter to ProtoUtils (that can call `ProtoLiteUtils` for now). It'd be an ExtensionRegistry instead of ExtensionRegistryLite. But at the same time `ExtensionInfo` doesn't provide much more info than a instanceof would provide, and it means going through the immutable/mutable weirdness in the API.