Add EscapingExamples

[mdbooth]

Adds a set of example which demonstrate the effects of escaping when generating validation rules. Specifically:

• `` vs "" for rule quoting
• "" vs r'' in CEL expressions

These tests justify the following general recommendations to avoid complex escaping:

• Always prefer `` for validation rules
• Always prefer r'' for strings in CEL expressions

[openshift-ci-robot]

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[openshift-ci]

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[openshift-ci]

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[openshift-ci]

@mdbooth: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/lint	73fa333	link	true	/test lint

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[mdbooth]

@JoelSpeed The linter is rejecting because apparently Pattern is banned in this repo. That obviously make this example impossible to write.

However, I also don't think it's a good idea. Certainly CEL has a lot of validation functions which we should require folks to use in preference to custom patterns. And as discussed, format markers in preference to those with the security related carve-outs documents in #2574. But if you're validating something that doesn't follow a standard kubernetes pattern then using Pattern is clearer and more concise than doing the same thing in a CEL expression. Quite apart from anything else, as demonstrated in this PR there are fewer escaping-related footguns.

A quick grep for Pattern in this repo shows that most, but critically far from all, of them should not exist.

Not sure what to do with this. My recommendation would be to remove the prohibition on Pattern and replace it with a softer review requirement that it needs a good use case. If you really want to stick with a ban then I'd also have to update #2574 to document that, as well as removing it here.

[openshift-merge-robot]

PR needs rebase.

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