Allow per PR/branch deployment

I want to have ability to do isolated deployments per PR/branch. Every such deployment should be done to the same account.

When I try to describe such PR/branch deployment pipeline with CDK, there is no way to use `${{ github.ref_name }}` or `${{ github.event.pull_request.number }}` in stage name.

Ideally, I would like to pass `stageName` while calling `.addStage(...)` and make it dependent on variables that will be later accessible in Github Workflow during execution.

Is there any trick that would allow me to do per PR/branch deployments?