feat(*): port several PDK and shared llm module from ai-proxy-advanced feature

[fffonion]

Summary

This PR adds several PDK methods to allow ai-proxy-advanced plugin to do customized load balancing and failover.
It also moves the ai-proxy handler to a shared module so that both ai-proxy{,-advanced} plugins can reuse same codebase.

https://github.com/Kong/kong-ee/pull/9562

Checklist

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

Issue reference

AG-12

[liverpool8056]

It seems that the retry_callback can be invoked only when balancer == nil, is it expected to work like this?

In addition, should we wrap retry_callback with pcall?

[fffonion]

It seems that the retry_callback can be invoked only when balancer == nil, is it expected to work like this?

Yes, if we skip the balancer lookup by doing kong.service.set_target, i.e. currently request doesn't have a upstream, then balancer, upstream = balancers.get_balancer(balancer_data) will return no balancer. Actually, this else branch is just made to support kong.service.set_target.
This matches our expectation, retry_callback will not be called if Kong's balancer is not skipped.

In addition, should we wrap retry_callback with pcall?

yes, adding that now.

[fffonion]

I will address all comments in EE side of PR and port here

[fffonion]

fixed

[fffonion]

Reviewers: please review https://github.com/Kong/kong-ee/pull/9562 first for a whole picture, I will then address comments there and port here.

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-13347-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-13347-to-master-to-upstream
git checkout -b cherry-pick-13347-to-master-to-upstream
ancref=$(git merge-base 982143769aa5bd271307fa70840051da1ca32a08 360b4f0e7d8a7d2a7e1ca373fd4f7d43a76b37e1)
git cherry-pick -x $ancref..360b4f0e7d8a7d2a7e1ca373fd4f7d43a76b37e1

[fffonion]

ce-ee cherrypick not needed, it's part of the ai-proxy-advanced PR and is merged