Introduce config file for cainjector options

[inteon]

Similar to #5337, but for the cainjector.
The approach and structure is identical to the existing setup for the controller and for the webhook.

Kind

/kind feature

Release Note

CAInjector config can now be loaded from a config file.

[SgtCoDFish]

/lgtm
/approve

This is so big that it's kinda hard to review. Part of that is our pattern of using k8s resources to define config rather than a much simpler format - that feels a bit like using a sledgehammer on a nail, but obviously it's the pattern we've established already so it makes sense to use it again here.

This is too big for me to be really confident about it, but I don't think that should be a blocker for us if tests pass, and I like the tests in cmd/cainjector/app/cainjector_test.go !

Thanks!

[jetstack-bot]

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