refactor(cloudformation): extract "custom-resources" module

[eladb]

Extract the AwsCustomResource construct to a separate module called @aws-cdk/custom-resources, dedicated to special custom resources such as this one.

Also, stop bundling the entire AWS SDK and instead extract apis/metadata.json during build time which fixes #2951.

BREAKING CHANGE: The AwsCustomResource class was moved to a new module called @aws-cdk/custom-resource

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[eladb]

@jogold FYI

[mergify]

Thanks so much for taking the time to contribute to the AWS CDK ❤️

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checklist:

• PR title type(scope): text
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