fix(jwt): remove chances to base64 decode a rsa_public_key

[jizhilong]

Summary

never base64 decode a rsa public key

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• [ ] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #13710

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[gszr]

Thanks for your contribution, @jizhilong! The fix looks good to me. @bungle could you also take a look?