[objc] Enable class name customization via RegisterAttribute
As a library creator, I want to enable fine grained control over the public API that I present so that my users find my library intuitive and easy to use. One aspect of this is conforming to naming conventions in the objc/Cocoa community. Xamarin.iOS's RegisterAttribute provides a natural vehicle for this, objcgen just needs to honor it.
I duplicate filed this here:
https://github.com/mono/Embeddinator-4000/issues/612
A rough impl draft was here: https://github.com/mono/Embeddinator-4000/pull/366
https://github.com/mono/Embeddinator-4000/issues/483 is a duplicate as well
with a different patch:
https://github.com/mono/Embeddinator-4000/pull/482