Example of op with a resource or custom object

[maedoc]

It'd be helpful to add an example use of ResourceMgr for third party C++ objects used. I put together a minimal example for my own purposes

https://gist.github.com/maedoc/37202e3e1b3dbff1b77dc6106e11917a

but it'd be great for the TF repo to have something, since it's not in the 'Create an op' tutorial.