Add a fake pending post-migrations response

What does this MR do and why?

We're on our way to test the independent post-deploy pipeline, most of the scenarios (that can be seen on gitlab-com/gl-infra/delivery#2352 (closed)) require pending post-migrations to be available. Post-migrations are not on the Delivery team control and can be time-consuming to wait for them.

Instead of waiting for them to be available, this commit adds fake post-migrations when a feature flag is enabled. This will allow us to test the initial scenarios (Phase 1) in a promptly manner.

This workaround should be removed once the testing has been completed.

Author Check-list

• [-] Has documentation been updated?