Introduce new state for GSI indexes that are scheduled for background creation

With MB-32238, the index creation request from the user will not be rejected, if there is already ongoing index creation. But in such cases, index will be scheduled for background creation. These indexes will be exposed via system:indexes so that user can track them.

But the state of these indexes cannot be mapped to any of the existing states. So, there is a need for a new state - "scheduled for background creation".