Draft: Introduce gen-facade.py script and demonstrate it with AttributeProxy

This MR is just to demonstrate my idea about how to pimpl classes using this gen-facade.py script. My hope is that each commit is understandable, and that we can build and run the tests for each commit.

I am just using AttributeProxy as an example here, and it perhaps isn't the best as the class isn't referenced in our external API for other classes so we didn't have to deal with that.

I might have a go at a more complicated example later.