Provide an example plugin implementation that embeds an upstream plugin

[evan2645]

It's common to want to provide custom selectors during node attestation without wanting to fork existing upstream plugins. The recommended way to do this is to embed the upstream plugin (i.e. wrap it), and inject custom selectors when Attest is called. That said, we don't have any good examples of how to do this.

This repository should provide, at a minimum, some docs and examples on how to do this.

[zmt]

I wrote a fairly generic node attestor wrapper plugin. I can probably clean it up to get slightly better config. shape and remove any employer-specific bits.