cli: pre-release integration test strategy for downstream tools (e.g. Amplify)

[mrgrain]

Today, any tools depending on the AWS CDK are forced to wait for a CDK release before they can execute their test suites against the latest version of the CDK. This is bad, because a change in the CDK might unintentionally break the other tool. Instead we should establish a pattern to test against pre-builds (nightlies) from main.

Primary concern will be for 1st party tools, however this could be extended to 2nd and 3rd party tooling as well.

[mrgrain]

Partially implemented by #31921

[iliapolo]

These types of test introduce the chance of bug in a downstream tool to cause delays in our release. We should have a mechanism to skip a integ test to free up a release, we want a fast mechanism that still requires 2 person review.