fix(apigateway): update imported RestAPIs type to grant access to RestAPIBase methods

[lpizzinidev]

fromRestApiId and fromRestApiAttributes were returning the imported RestAPI as IRestApi, preventing methods implemented by RestAPIBase from being called without type casting.
This fix solves the problem by changing the return type of the two functions.

Closes #26247.

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[gitpod-io]

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[lpizzinidev]

Exemption Request.
Let me know if integration tests or other changes are needed.

[aws-cdk-automation]

The pull request linter fails with the following errors:

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[mrgrain]

Hey thanks for the PR, unfortunately we can't go forward with this PR like this. Here's why:

*Base types are not considered "public" and should not be used as return or property types.
I appreciate that in the past we might have not been consistent with that.

You are welcome to open a new PR with a different approach. As this is a p2 issue, we cannot guarantee to have time to work with you on an alternative solution. Please reach out to the wider community for support.