Support source code repository multiple deployments in sequence

**Tell us about your request**
When using App Runner configured to trigger deployments through Source code repository, if you push multiple commits in parallel to the target branch and the first commit triggered the app deployment, the other commits are not put in a queue for further deployments. If the first deployment related to the first commit is under "operation in progress", the subsequent commits/changes to the source code are not deployed.
App Runner should have a mechanism to put these changes in a queue for further deployments.

Deployments configured to use container images work properly if you push multiple images to the registry and the app runner service is under "operations in progress". We should follow that approach for source code repository as well.