feat(aws-ec2) export NatGatewayProvider for consistency with NatInstanceProvider

[crh23]

Export NatGatewayProvider for two reasons: to allow instantiation with new, and to make docs present it as an implementation of NatProvider.
Also added a unit test for the same "functionality" for NatInstanceProvider, for symmetry with the added test for NatGatewayProvider.
Added an integration test, because it appears that the docs pull from them for examples (I don't know how to test this).

Closes #28372.

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[aws-cdk-automation]

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[aws-cdk-automation]

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[crh23]

It turns out that I did not successfully run an integration test. Can't get them to work locally for some reason, I'll try again next year.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: f2be2e0
• Result: FAILED
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