fix(cloudfront): China regions do not support CachePolicy

[pahud]

Still clarifying some details internally on V1476577865.

AWS China regions do not have CachePolicy support yet the defaultBehavior would always render a behavior with CachePolicyId. This PR checks if the stack is deploying to a China region and optionally disable the CachePolicyId and add a dummy forwardedValues prop, which is required if CachePolicyId undefined per CFN doc.

As _renderBehavior() has to be executed in CDK synth time, the only way to check if we are deploying to an unsupported region is from the stack region env hence we note this requirement in the README.

Issue # (if applicable)

Closes #31033 and #13584

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• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[pahud]

Exemption Request

This PR does not need to update the integ tests.

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