Add support for drop-in config

[harche]

Helps in use cases where a drop-in config could help by allowing users to update a subsection of the config instead of touching an entire config file.

e.g.

1. https://github.com/containers/storage/issues/1306
2. https://github.com/openshift/enhancements/pull/1600/files#diff-10bfa811cdabc678cbfd947dbeaf04b63ebd24e8d9ab5a05c7abbb998cb72f8aR135
3. https://github.com/kubernetes/enhancements/issues/4191

[openshift-ci[bot]]

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[harche]

/hold

[rhatdan]

Could you open a [WIP] PR against Podman to test this feature, and make sure it does not break anything in it's ci/cd system.

[harche]

Could you open a [WIP] PR against Podman to test this feature, and make sure it does not break anything in it's ci/cd system.

@rhatdan I opened https://github.com/containers/podman/pull/22484. Most of the jobs were green, apart from the one for the mac. I am not able to trigger the retest of that job, it needs /ok-to-test label.

[rhatdan]

@nalind @giuseppe @saschagrunert @vrothberg PTAL

[cgwalters]

This would be really quite useful to have, it's just completely annoying right now to e.g. enable composefs because one needs to copy all of the storage config.