Validation webhook: use error list while validating `PolicyServer` CRD

[flavio]

Currently the validation webhook implemented by the kubewarden-controller will reject the creation/update of a PolicyServer resource as soon as it finds something that is not valid.

We should instead populate a list of all the non-compliance issue of the analyzed PolicyServer resource and then return all of them while rejecting the creation/update.

This would improve the UX: the user would get all the "wrong things" immediately, instead of having to iterate over each one of them (loop: apply - > reject -> edit -> apply over and over).

Admission criteria

• Accumulate list of violations inside of field path, see here
• Use error list like done here
• make use of more functions/constants provided by apimachinery, like apimachineryvalidation.IsNegativeErrorMsg