feat(rds): add metricReadIOPs method to DatabaseClusterBase

[jasdeepbhalla]

Issue #35327

Closes #35327.

Reason for this change

Currently, DatabaseClusterFromSnapshot only exposes the metricVolumeReadIOPs() method for monitoring cluster-level I/O metrics. Users cannot easily track instance-level ReadIOPs at the cluster level, which is required for compliance monitoring (e.g., Drata) and simplifies CloudWatch alarm configuration.

Description of changes

• Added metricReadIOPs(props?: cloudwatch.MetricOptions): cloudwatch.Metric to DatabaseClusterBase.
• The new method returns a CloudWatch metric for instance-level ReadIOPs aggregated across all instances in the cluster.
• Available on both DatabaseCluster and DatabaseClusterFromSnapshot by inheritance.
• Follows the existing pattern used for other metrics such as metricVolumeReadIOPs and metricACUUtilization.
• Allows users to create alarms or dashboards using the ReadIOPs metric at the cluster level without manually configuring metrics for each instance.

Example usage:

const cluster = DatabaseClusterFromSnapshot.fromSnapshot(...);

const readIOPSMetric = cluster.metricReadIOPs({
  period: cdk.Duration.minutes(5),
  statistic: 'Average'
});

new cloudwatch.Alarm(this, 'ReadIOPsAlarm', {
  metric: readIOPSMetric,
  threshold: 1000,
  evaluationPeriods: 1,
});

Describe any new or updated permissions being added

• No new IAM permissions are required, as this change only exposes a CloudWatch metric for monitoring purposes.

Description of how you validated changes

• Updated Integration test to verify that metricReadIOPs returns a CloudWatch Metric with the correct metric name and dimensions.

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

[aws-cdk-automation]

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[jasdeepbhalla]

@pahud This PR is ready for review!