fix(apigateway): move url property to RestApiBase

[hariprakash-j]

Description of the fix

Moved the url property from RestApi class to the RestApiBase to have the url property on all derived class. This will fix the problem mentioned in the issue where SpecRestApi does not work with the RestApiOrigin construct.

PS. I could not perform the integration tests as the free tier on my personal account expired.

Closes #27501.

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[aws-cdk-automation]

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[hariprakash-j]

Exemption Request

I just moved the property to the base class, I am not sure what tests are required for this. If there is any thing that I can do for this please let me know.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 0c88ca9
• Result: SUCCEEDED
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[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to a test file.
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[mergify]

⚠️ The sha of the head commit of this PR conflicts with #27742. Mergify cannot evaluate rules on this PR. ⚠️