Add plugin hook for adding new load balancing algorithms

[airhorns]

Currently, it doesn't seem possible for a plugin to extend the available load balancing algorithms that can be used. Instead, operators needs to fork, rebuild and deploy the project, or ninja-mount lua files, overwriting source code at runtime, both of which seems error prone and hard.

But, there's lots of interest in new or different load balancing algorithms for ingress-nginx, a lot of which haven't made it into the core. #9239 and #9025 are both attempts to add new algorithms that could have been plugins. Personally, I have all sorts of weird ideas I want to try for my own load balancing problems, and I'm also aware of several different private initiatives that do the same.

It'd be great if we could add new implementations to the list of available implementations, and then access all annotations inside a plugin to power it.

And finally, for bonus points, its really hard to test plugins outside this repo, it'd be really great to improve support for that as well.

[k8s-ci-robot]

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[github-actions]

This is stale, but we won't close it automatically, just bare in mind the maintainers may be busy with other tasks and will reach your issue ASAP. If you have any question or request to prioritize this, please reach #ingress-nginx-dev on Kubernetes Slack.