fix(appset): allow cluster urls to be matched (#13715) (2.6 cherry-pick)

[blakepettersson]

• fix: allow cluster urls to be matched

Related to #13646, and after discussion with @crenshaw-dev, it turns out that matching on cluster urls is not possible. This is due to the fact that the implementation of LabelSelectorAsSelector from k8s.io/apimachinery validates that a label value is no longer than 63 characters, and validates that it's alphanumeric. In order to work around that, we'll create our own implementation of LabelSelectorAsSelector.

This implementation has been copied verbatim, with the difference that in isValidLabelValue, we first check if the label value is a valid url. If it is not, we proceed with the label checks as with the original implementation.

Apart from that, the only other differences are making as much as possible to be package-private; the intent is to only make Matches and LabelSelectorAsSelector available from outside the package.

• chore: drop all label value restrictions

We want to be more flexible in what we accept in post-selectors, mainly that we want to allow other values than only server urls. For this, we will drop all restrictions that a typical "label value" would typically have.

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