(pipelines): ManualApprovalStep should support SNS notifications

### Description

`pipelines.ManualApprovalStep` should be able to send an SNS notification to a user when their approval is requested.

### Use Case

I'm migrating an old `pipelines.CdkPipeline` to `pipelines.CodePipeline`. The previous pipeline is using `codepipeline_actions.ManualApprovalAction` which supports the above functionality, but the parallel `pipelines.ManualApprovalStep` does not.

### Proposed Solution

`pipelines.ManualApprovalStep` exposes an SNS notification topic property for providing this configuration.

### Other information

As a workaround, one could implement a custom `Step` that exposes the `codepipeline_actions.ManualApprovalAction` for the `pipelines.CodePipeline` version.

### Acknowledge

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

Start by comparing the implementation and usage of pipelines.ManualApprovalStep with codepipeline_actions.ManualApprovalAction and inspect how SNS notification topics are configured. Trace the pipelines.CodePipeline path to identify the relevant construct entry point and tests. Done means ManualApprovalStep exposes the requested SNS topic configuration and its behavior is covered by the project’s existing tests.

Written by the indexing model from the issue text.