Validation: add documentation and use CEL pre-processor

[howardjohn]

Depends on istio/tools#3050.

This PR adds support for a new cel pre-processor: https://github.com/howardjohn/celpp.

This avoids the need to write incredibly verbose and error prone boilerplate to workaround limitations in the CEL language.

---

This PR has a few commits:

• Run with no changes. The pre-processor formats the CEL expression, so this has some trivial changes
• Incrementally migrate things to the new CEL macros where it makes sense to simplify
• Add documentation for validation (includes more than just the new stuff)

[istio-testing]

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

Test name	Commit	Details	Required	Rerun command
gencheck_api	584cc10	link	true	/test gencheck
release-notes_api	584cc10	link	false	/test release-notes
build_api	584cc10	link	true	/test build

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