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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