feat(lambda): add domainName prop to FunctionUrl

[mbfreder]

Issue # (if applicable)

Closes #23005.

Reason for this change

This change is to address this issue

Description of changes

This PR adds a new property called domainName to the FunctionUrl construct. The goal is to make it easy for customers to get the domain name directly (FunctionUrl.domainName) instead of first getting the url (FunctionUrl.url) and then removing the scheme and the trailing slash.
The README has been updated.

Description of how you validated changes

I Added some unit tests. I Don't think integration tests are necessary in this case.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 88a4da2
• Result: FAILED
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[mbfreder]

I don't think integration tests are needed in this case.