support comma in string pars value

[shuqz]

Issue

#4363

Description

• escape comma in string map parsing logic (we are having same escape pattern for gateway multi header value, so decide to be consistent here)
• update doc for instructions

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

 alb.ingress.kubernetes.io/listener-attributes.HTTPS-443: routing.http.response.access_control_allow_headers.header_value=testHeader\,anotherNewValue

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Doc update

• 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]

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