fix(api-gateway): add validation to variables property on Stage resource

[sumupitchayan]

Closes #3635

From the original issue: API Gateway Stage variables must match a regex to be valid (See the documentation). When adding variables using CDK, this regex is not enforced, so it can synthesize a CloudFormation template that cannot be deployed.

This PR fixes this issue by adding a validation that the strings match that regex along with a unit test for these changes.

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[gitpod-io]

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 811e599
• Result: SUCCEEDED
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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).