feat: support cfn outputs

[kaizencc]

This PR adds support for cfn outputs. You can specify a Cfn Output as an environment variable like so:

pipeline.addStage(myStage, {
      pre: [new ShellStep('Pre', {
        commands: ['echo hello'],
      })],
      post: [new ShellStep('Post', {
        envFromCfnOutputs: {
          FN_NAME: myStage.fnName,
        },
        commands: ['echo FN_NAME equals: $FN_NAME'],
      })],
    });

This translates to a deploy.yml file with an output for the deploy step:

    outputs:
      myout: \${{ steps.Deploy.outputs.myout }}

As well as sets the environment variable for the Post step:

    env:
      FN_NAME: \${{ needs.StageA-FunctionStack-Deploy.outputs.myout }}
    steps:
      - run: \\"echo FN_NAME equals: $FN_NAME\\"

[kaizencc]

If someone can figure out why I depend on StageA-FunctionStack-Deploy twice, that would be great :).

[kaizencc]

This looks to be a bug on the pipelines construct, and does not impact the performance (just looks ugly)

[kaizencc]

Tracking: aws/aws-cdk#18450