Implement promotion phase in KeptnAppVersion controller

[mowies]

Overall Goal

As a Keptn user, I want to have a dedicated way to promote my GitOps enable app to the next stage in my environment.

Details

Implement the promotion phase controller logic.
The promotion phase should be run after the post deployment and post evaluation tasks are finished.
The promotion phase should only be execute if post deployment and post evaluation phase succeeded.

DoD

• implementation
• unit tests
• integration tests

Dependencies

• Enhance KeptnAppContext and KeptnAppVersion with promotion task list and status fields #3017
• Create Helm feature flag for promotion feature #3035