CAR - 2 eng weeks for backporting to 8.0

Implementation Sketch

• Add mode to the ShardRegistry to only contain one entry and spawn RSM for the `config` shard; no reads against the config.shards collection.
• Add mode to the CatalogCache to only return entries for the admin.* and config.* collections; no reads against the catalog collections.
• Add logic to the injector to report lastOpCommitted in its $replData so that the standby replica set's notion of the majority committed optime advances. The oplog shipper reads from the active cluster with rc:majority so it is always valid for these optimes to be advertised as majority committed.