Add License Linting

[maxsmythe]

Describe the solution you'd like
CNCF limits what licenses we should be using for vendored libraries: https://github.com/cncf/foundation/blob/main/allowed-third-party-license-policy.md

Here is how K8s automates validating compliance: https://github.com/kubernetes/kubernetes/blob/master/hack/verify-licenses.sh

To reduce risk of accidentally vendoring in non-compliant code, we should do something similar.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Gatekeeper version:
• Kubernetes version: (use kubectl version):

[stale]

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