[AWS::Events::Rule] - [Enhancement] - Add SnsParameters support for Target property to enable SNS message attributes

### Name of the resource

AWS::Events::Rule

### Resource name

_No response_

### Description

The `AWS::Events::Rule` Target property currently supports service-specific parameters for several target types (`EcsParameters`, `SqsParameters`, `KinesisParameters`, `BatchParameters`, `HttpParameters`, `RedshiftDataParameters`, `RunCommandParameters`, `SageMakerPipelineParameters`, `AppSyncParameters`), but does not include `SnsParameters` for SNS targets.

When using EventBridge rules to deliver messages to SNS topics, there is no way to specify SNS `MessageAttributes`. This is a limitation at the EventBridge API level (`PutTargets` does not support `SnsParameters`), which propagates to CloudFormation.

SNS MessageAttributes are essential for SNS subscription filter policies. Without them, we have to use a Lambda function as an intermediary between EventBridge and SNS to set message attributes — adding cost, latency, and operational overhead for what should be a direct integration.

**Requested change:**

Add an `SnsParameters` property to the `Target` structure of `AWS::Events::Rule`, supporting at minimum:
- `MessageAttributes` — key/value pairs with `DataType` and `StringValue`, consistent with the SNS `Publish` API

This requires the EventBridge service API to first add `SnsParameters` to the `Target` structure in `PutTargets`.

### Other Details

- Related CDK feature request (labeled `needs-cfn`): https://github.com/aws/aws-cdk/issues/35875
- EventBridge Target API reference (no SnsParameters): https://docs.aws.amazon.com/eventbridge/latest/APIReference/API_Target.html
- CloudFormation Target property reference (no SnsParameters): https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-events-rule-target.html

Start with the EventBridge Target API reference and verify whether PutTargets supports SnsParameters; the issue identifies this service API gap as a prerequisite. Compare the related AWS CDK feature request, then confirm that CloudFormation’s AWS::Events::Rule Target reference can expose MessageAttributes once the upstream API supports it.

Written by the indexing model from the issue text.