Extract and expose evaluation/assertion logic for checks

[komish]

Problems

• In several places, we bind tasks that are important for our execution to tasks that perform data verification, comparison etc.

• Another example might be test cases, where we need to write information to a file (say, as an artifact), and we first must verify certain elements of the data we need to validate before we write it.

• In some cases, we also store this logic behind our internal packages, in our checks themselves.

• There’s value, at the very least from a testing perspective, and beyond that - to users who may want to perform preflight checks within their own test code, to decoupling the logic behind validating a check’s criteria against raw data, and performing other logic.

The solution should:

• Expose functionality that accepts data structures and evaluates our check conditions as library functions.
• Decouple our runtime logic from this validation

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One of the simplest examples (and therefore one of the locations where we’d get the least amount of value, arguably) would be the HasRequiredLabels check.

https://github.com/redhat-openshift-ecosystem/openshift-preflight/blob/main/certification/internal/policy/container/has_required_labels.go#L31-L48

This validation could effectively be a public function that we then call in this method.

Similar/Related: #639

[acornett21]

When #639 was raised I was thinking maybe we need an api or pkg package that calls our Validate function of each check. That way our validation code, need not be exposed fully and we can still rely on an internal package. There is probably more work for this issue 642, but maybe having a public layer like I mentioned would be also helpful.

[komish]

Marking this issue as stale, and also in favor of #639, which is opened by the community and will be a more collaborative discussion.