fix(cognito): getting user pool client secret creates unnecessary custom resource

[laurelmay]

This updates the UserPoolClient construct to not use a custom resource to fetch the secret. I thought about gating this behind a feature flag (and am still happy to do so) but this should functionally behave the same (the underlying value doesn't change, just the token) and it doesn't require any code changes for end users, it just removes a now-unneeded construct.

Without generateSecret: true the ClientSecret attribute is not available so those checks are still retained (as are the related tests).

The ClientSecret attribute seems to have been available in the resource schema for quite awhile but it isn't documented on the CloudFormation resource page.

Closes #28785.

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[laurelmay]

Woo! Not sure why the integration testing didn't replace the hosted zone and domain correctly... I guess I'll try again.

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