Cloudwatch: Support live data for SingleValueWidget in CloudWatch dashboards

### Describe the feature

Currently, the `SingleValueWidget` in CDK-defined CloudWatch dashboards does not support live or auto-refreshing data. This limits its usefulness for operational monitoring of key metrics (e.g., errors, throttles, queue depth). Other widget types support near-real-time updates, but SingleValue widgets cannot be configured for this behaviour via CDK.

### Use Case

I want to display a SingleValue widget on a dashboard that **updates automatically** to reflect near real-time metrics without manually refreshing the dashboard. For example:

```ts
import * as cloudwatch from 'aws-cdk-lib/aws-cloudwatch';
import * as cdk from 'aws-cdk-lib';

const dashboard = new cloudwatch.Dashboard(stack, 'MyDashboard');

dashboard.addWidgets(
new cloudwatch.SingleValueWidget({
metrics: [
new cloudwatch.Metric({
namespace: 'MyApp',
metricName: 'ErrorCount',
period: cdk.Duration.minutes(1),
}),
],
}),
);
```

Currently, this widget only shows a static snapshot of the metric, even if the dashboard refreshes.

### Proposed Solution

Allow `SingleValueWidget` to optionally display live or near-real-time data.

### Other Information

_No response_

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

2.237.1

### AWS CDK CLI version

2.1101.0

### Environment details (OS name and version, etc.)

Mac OS Sonoma

Start at the SingleValueWidget entry point in the aws-cloudwatch library and compare how other widget types support live or near-real-time data. Identify the optional configuration needed for a live SingleValue widget and verify its rendered CloudWatch dashboard definition with the relevant widget tests; done means the setting is available through CDK and produces the expected dashboard behavior.

Written by the indexing model from the issue text.