refactor: more generic target handling

[erikgb]

In this PR I am trying to make the code for handling targets (configmaps and secrets) even more generic. My initial goal was to fully remove the code duplication for configmaps/secrets, but I do see (at least not now) how that can be done. I am suggesting merging this as a step forward for now.

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[inteon]

As you said, this PR does not reduce/ deduplicate code yet. I'm ok with merging this if you think this will bring us closer to a refactor where it will be possible to deduplicate the code. /lgtm /approve

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[erikgb]

@inteon The PR is rebased now. Need a new LGTM.

[inteon]

/lgtm