validations: add pattern field validation

[LoginovIlia]

What this PR does

Adds validation for the pattern field when a pattern is added or changed. Removing a pattern is still allowed

Rationale

Changing or adding a regex may block values that worked before, so we mark these changes. Removing a regex makes the input more open, so we do not mark it

Changes

• Adds validation in pkg/validations/property/pattern.go with unit tests
• Adds a new section to docs/validations.md
• Adds e2e tests: test/patternadded and test/patternchanged

[k8s-ci-robot]

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This pull-request has been approved by: LoginovIlia
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[k8s-ci-robot]

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[LoginovIlia]

/cc @jpbetz

[everettraven]

Just a heads up that this is on my list to take a look at soon. If I don't get to it today, it will be on my list for first thing next week. Thanks for contributing!

[everettraven]

Overall, this looks really good - thanks for taking the time to contribute!

I'm requesting changes here because of some recent conversations I've had with folks that brought to my attention the importance of taking into account how loosening validations may impact both readers and writers of an API.

Because crdify doesn't have nuanced knowledge of a situation, we should add a knob here so that users of crdify that do not want to allow removal of a pattern constraint can do so and we should default to that behavior (we always default to the most restrictive at the moment).

[everettraven]

We may want to allow users to restrict removal of a pattern as well (and maybe we default to restricting this too).

Removing a pattern is a compatible change for writers, but is likely a breaking change to readers.

Older clients may no longer be able to handle the values here if they were built on the assumption that valid values for that field always adhere to that pattern.