refactor(pdk): output content with string.buffer

[chronolaw]

Summary

Use string.buffer to replace slow table.concat.

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

Full changelog

• [Implement ...]

Issue reference

Fix #[issue number]

[chobits]

hi @chronolaw need one performance test result, maybe a simple and informal test like some loops on both of these two implementation (wrap them into some test function), otherwise it looks ok

[chronolaw]

Aapo did some tests about string.buffer and table.concat. Please refer to it. #11304 (comment)

[chobits]

Approve, I have checked the refactoring logic, confirm it works as expected

[chobits]

Aapo did some tests about string.buffer and table.concat. Please refer to it. #11304 (comment)

Pretty cool. I think the later reviewing for similar PR like refactoring table.concat with string.buffer will be easier.