(pipelines): Confirm permissions broadening before SelfMutate action

As a security enhancement, give users the option to include a CodePipeline approval step prior to letting the pipeline mutate itself (and changing its security context).

Essentially, the mechanics should work exactly like the existing [confirm-permissions-broadening](https://docs.aws.amazon.com/cdk/api/latest/docs/pipelines-readme.html#confirm-permissions-broadening) approval gate.

The difference is rather than checking for target environment security changes, the new option would check for changes in the pipeline's security context (pipeline roles, security groups, etc.).

### Use Case

This feature would enhance security, by ensuring any change in the pipeline's security context must be approved by the right set of people.

### Proposed Solution

If possible, I would suggest exposing the UpdatePipeline stage (`Stage` and `StageDeployment` instances). This way, we could do:

```ts
pipeline.updatePipelineStageDeployment.addPre(
new ConfirmPermissionsBroadening('Check', { stage: pipeline.updatePipelineStage })
);
```

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

---

This is a :rocket: Feature Request

Start by reading the existing ConfirmPermissionsBroadening mechanism and the proposed updatePipelineStageDeployment, Stage, and StageDeployment entry points. Determine how a pre-approval gate would identify changes to the pipeline's security context; done means users can opt into approval before SelfMutate changes that context, with behavior matching the existing approval gate.

Written by the indexing model from the issue text.