WIP: Add CRDs and controller for execution-hook

[xing-yang]

This PR contains POC code that adds CRDs and controller for execution-hook.

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[nikhita]

tiny nit: the boilerplate header needs to have extra newlines.

For reference: https://github.com/kubernetes/kubernetes/blob/master/hack/boilerplate/boilerplate.go.txt

[xing-yang]

Sure. Thanks @nikhita!

[xing-yang]

Close this PR now in favor of #3.