Feature: Add support for Gatekeeper Add-On

[aruanurag]

What does this PR do?

Installs gatekeeper as a helm add-on

🛑 Please open an issue first to discuss any significant work and flesh out details/direction - we would hate for your time to be wasted.
Consult the CONTRIBUTING guide for submitting pull-requests.

Motivation

gatekeeper is the new recommended was of introducing policy as code. Since PSP is getting deprecated eventually everyone will be using Gatekeeper for runtime security of the k8s cluster

• Resolves [ADDON] - OPA #434

More

• Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• Yes, I have added a new example under examples to support my PR
• Yes, I have created another PR for add-ons under add-ons repo (if applicable)
• Yes, I have updated the docs for this feature
• Yes, I ran pre-commit run -a with this PR

Note: Not all the PRs require a new example and/or doc page. In general:

• Use an existing example when possible to demonstrate a new addons usage
• A new docs page under docs/add-ons/* is required for new a new addon

For Moderators

• E2E Test successfully complete before merge?

Additional Notes

Test example: https://www.eksworkshop.com/intermediate/310_opa_gatekeeper/policy-example-1/

[bryantbiggs]

looks good - we'll also want to add a PR to support the GitOps route here https://github.com/aws-samples/eks-blueprints-add-ons

[bryantbiggs]

Looks great, thank you for the contribution!

Please don't forget to add this to the GitOps addons repo, you can see an example of how to add here aws-samples/eks-blueprints-add-ons#72