feat(pdk): add service.set_proxy_address function

[locao]

Summary

Add a new PDK function to set the IP and port to proxy the request, overwriting the result from the load-balancer.

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

KAG-5628