aws-pipelines: add `executionMode` property to `CodePipeline` construct

### Describe the feature

Add `executionMode` property to `CodePipeline` construct. The same that exists in the L2 construct Pipeline in aws-codepipeline

### Use Case

The L2 codepipeline construct has `executionMode`, would be super useful to have it in the L3 construct aswell.

### Proposed Solution

Same as https://github.com/aws/aws-cdk/pull/29148 but for the aws-pipeline CodePipeline construct

### Other Information

Maybe add this to the JSDocs

https://docs.aws.amazon.com/codepipeline/latest/userguide/concepts-how-it-works.html

### 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)

2.206.0

### AWS CDK CLI version

2.1021.0

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

MacOS 15

Start at the aws-pipelines CodePipeline construct and compare its API with the L2 Pipeline construct and the approach in PR #29148. Read the AWS CodePipeline execution-mode documentation linked in the issue. Done means the L3 construct exposes the executionMode property consistently, with appropriate JSDoc and coverage for the supported behavior.

Written by the indexing model from the issue text.