Support --dump-config option

[soupdiver]

It would be nice to support a flag like --dump-config that dump the final config on startup. Even/Especially if it's invalid.

I know there is --v=3 but that also prints out a whole lot of other things. I was mainly interested in the config for debugging purpose.

I raised in issue here:
#12330

Some context:
I did maintenance on a cluster with a bunch of ingresses and apps and it's not entirely clear where all the ingrresses come from but they are there. One of them is affected by #11866 and it was tedious to find out from which ingress the error came.
I used --v=3 but that dump not valid json I could use. It just looks like almost json. I also went into the container to get the config but no luck either. The startup process is pretty integrated from what I saw in the code.
A tmp config is written and checked and if invalid startup aborts and the file is deleted. Without touching the code I could not find an easy way to just the config file.

I think the dump I had to scroll through was 2MB of unformated text.

[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.