pipelines: Stages to support max concurrent steps

### Describe the feature

When using stages with pipelines, there is currently no way to define desired concurrency (akin to `cdk deploy --concurrency`). Adding a new optional parameter to `aws-cdk-lib » pipelines » Wave` of `concurrency` would direct stage creation to use N number of max concurrent steps.

### Use Case

Pipelines uses a large concurrency for stage creation. This causes issues when making larger number of changes to certain services, such as DMS. DMS starts erroring out when more than 8-10 of certain changes take place at the same time. Depending on the changes being implemented, this can cause rollback issues as well.

### Proposed Solution

Add optional parameter to `aws-cdk-lib » pipelines » Wave` of `concurrency`. This would then alter how many steps are added to each stage.

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.118.0

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

Python 3.12, Linux/macos

Start by locating aws-cdk-lib » pipelines » Wave and the stage-creation entry point in the TypeScript sources. Read the related pipeline tests, if present, to understand how steps are currently added; done means an optional concurrency setting limits the maximum concurrent steps without changing behavior when it is omitted.

Written by the indexing model from the issue text.