(CDK Pipelines): allow passing a Role to `CodePipeline`

### Description

Because of https://github.com/aws/aws-cdk/issues/16244, customers have to sometimes create a Role manually and control its permissions.

We should make that easy in `CodePipeline`.

### Use Case

See above.

### Proposed Solution

Add a new property `role` to the `CodePipeline` construct.

### Other information

_No response_

### Acknowledge

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

Start at the CodePipeline construct entry point and trace how its pipeline role is configured. Check the existing CodePipeline tests for coverage of construct properties; done when callers can pass a manually created role and CodePipeline uses it.

Written by the indexing model from the issue text.