fix(patchs): fix `kong.request.enable_buffering` PDK function cannot be used when dowstream uses HTTP/2.

[oowl]

Summary

1. kong.request.enable_buffering PDK function can work fine when downstream using HTTP/2, which also means all of the kong plugins including the response function phase now work in HTTP2 protocol.
2. Remove http2 hardcode limitation in ngx.location subrequest API by patch https://github.com/openresty/lua-nginx-module/pull/2355
3. Extend kong test helpers function helpers.proxy_ssl_client and helpers.proxy_clien http2 protocol support by https://github.com/oowl/lua-reqwest
4. Improve some of the http2 downstream protocol tests in our test cases. eg: spec/03-plugins/38-ai-proxy/* and spec/02-integration/05-proxy/24-buffered_spec.lua

Checklist

• [x] The Pull Request has tests
• [x] 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

AG-108 FTI-5725