Reduce duplicated DescribeTargetGroups calls

[wweiwei-li]

Issue

With the frontend NLB feature, we query the ELB TGs twice. One for FrontendNlbTargetSynthesizer, another for TargetGroupSynthesizer. which has resulted in increased API call volume

Description

This PR optimizes it by sharing the target group data between FrontendNlbTargetSynthesizer and TargetGroupSynthesizer to avoid duplicate API calls

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
• [x ] 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: wweiwei-li

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