Use child pipeline for review apps

What does this MR do and why?

Move review-app related jobs into a child pipeline. This simplifies the pipeline interface for anyone who needs to deploy the review app manually.

Instead of having multiple "review-app" jobs review-build-cng, review-deploy, review-qa-*, etc, in the main pipeline, one would only see start-review-app-pipeline job. This job triggers a child pipeline containing all the review app related jobs. The status of this job is dependent on the status of the child pipeline.

Main pipeline:

Child pipeline:

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #328378 (closed)