iot-actions-alpha: support batchAcrossTopics in HTTPS action batchConfig

### Describe the feature

Expose `batchAcrossTopics` on `HttpActionBatchConfig` in `@aws-cdk/aws-iot-actions-alpha`. The property is available in L1 (`CfnTopicRule.BatchConfigProperty`) but is not surfaced in L2.

### Use Case

Route messages from many MQTT topics to a single HTTP endpoint with one batched request, reducing per-request overhead and cost. See https://docs.aws.amazon.com/iot/latest/developerguide/http_batching.html

### Proposed Solution

Add `batchAcrossTopics?: boolean` (default `false`) to `HttpActionBatchConfig` and pass it through in `_bind()`.

### 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)

2.224.0

### AWS CDK CLI version

2.1041.0

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

macOS 14.6.0 (Darwin 23.6.0)

Locate HttpActionBatchConfig and its _bind() implementation in @aws-cdk/aws-iot-actions-alpha. Compare its output with the L1 CfnTopicRule.BatchConfigProperty, then verify that batchAcrossTopics defaults to false and is passed through; done means the L2 property is available and represented in the bound configuration.

Written by the indexing model from the issue text.