codeintel: Add disable indexer flag

Created by: efritz

Once we deploy the VM-based indexer, we'll need to ensure that we're not continuing to process indexes inside the cluster. This flag will help us in this transition:

• Process all indexes within cluster
• Deploy new VM-based service
• Disable indexing within cluster
• Start new VM-based service
• Check health; reset flag on issues, continue if the new service works well
• Remove in-cluster index code