Validation: add documentation and use CEL pre-processor

[howardjohn]

Depends on https://github.com/istio/tools/pull/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.

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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)