[pipelines] how to add notifications to a CDK pipeline? or: expose underlying CodePipeline object

As far as I can tell, there is no way with the current CDK pipeline API to add codestarnotifications to the underlying CodePipeline, because there is no way to get the ARN of the CodePipeline.

The CdkPipeline has a private variable that holds the reference to the CodePipeline, but we can't access it. I imagine that it would be useful for developers to have access to this for many other reasons as well.

### Use Case

There is no way to set up notifications for a CDK pipeline.

### Proposed Solution

Change CdkPipeline `private readonly _pipeline` member variable to public.

### Other

* [x] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start by reading the CdkPipeline class and its private readonly _pipeline member, then inspect how the underlying CodePipeline is constructed and exposed. The change is complete when callers can access the underlying pipeline and use its ARN to configure CodeStar Notifications.

Written by the indexing model from the issue text.