feat: Introduce annotation for setting explicit egress CIDR ranges on Service LB frontend security group

[mtweten]

Issue

Description

This PR implements a new annotation (service.beta.kubernetes.io/aws-load-balancer-outbound-cidrs) for Service load balancers to enable explicitly specifying a list of CIDR ranges to be added as egress rules to managed frontend security groups.

Currently, aws-load-balancer-controller doesn't set any explicit egress rules during SG creation, relying instead on AWS to create a default all-protocol 0.0.0.0/0 rule when no explicit egress rules are specified during SG creation. This egress rule is necessary for health checks on target groups, etc.

However, some organizations may have security scans that trigger on this 0.0.0.0/0 rule, and it would be desirable to be able to scope that down to a specific set of CIDR ranges instead (e.g. the VPC CIDR, for example). There is currently no mechanism in aws-load-balancer-controller to allow setting this, so that is what this is intended to provide.

Note that this only implements this for the Service load balancer (e.g. nlb) workflow. I wanted to keep the PR more narrowly scoped, but it would probably relatively simple to implement for other workflows as well. There is also the backend SG where a command flag would need to be introduced, but it's easy enough just to use a statically created SG for the backend SG using the existing mechanisms instead.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[linux-foundation-easycla]

• ✅login: mtweten / name: Michael Tweten / (c7cbffb)

The committers listed above are authorized under a signed CLA.

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