feat(aws-cloudwatch): add support for labels on pie charts (aws#28929).

[kofrasa]

Add a new "labels" property for graph widgets to show ratios for pie graphs.

Issue #28929

Closes #28929.

Reason for this change

When using a pie graph, the magnitude of ratios are not shown unless the { "labels": { "visible": true } } property is enabled.

Description of changes

The commits adds a new "labels" property with the structure LabelProps on the graph widget.
The structure contains a single nested boolean value "visible" which toggles the presence of the label on a pie graph. When set to true, CloudWatch dashboard will render the magnitude of ratios as percentages on the graph.
The README has been updated to reflect the new property and how it can be used.

Description of how you validated changes

The commit includes unit, integration, and snapshot tests as would be generated by the CDK library.
The change has also been manually verified from the CloudWatch console by enabling and disabling the property.

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-cdk-automation]

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: e69dc59
• Result: FAILED
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[mergify]

⚠️ The sha of the head commit of this PR conflicts with #30287. Mergify cannot evaluate rules on this PR. ⚠️