[feat] allow multiple targetgroupbindings to reference same tg arn if using multicluster mode

[zac-nixon]

Issue

n/a

Description

Users would like to utilize the same Target Group ARN for multiple TargetGroupBindings. Previously, before Multicluster support, this was not possible as the reconcile logic would be unaware of other entities registering targets into the configured Target Group. Now, using Multicluster TargetGroupBindings, the reconcile logic is able to only operate on targets in the Target Group that belong to the specified TargetGroupBinding.

We can relax the validation logic to allow users to specify duplicate Target Group ARNs if they also specify the Multicluster flag on each binding.

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 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zac-nixon

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[zac-nixon]

/ok-to-test