[RW Separation] Introduce new search replica concept and changes to create API

[mch2]

For this first task for rw split we need to update shardRouting to include a flag indicating the allocated shard is a search only replica. We should be able to use this to update routing tables and add/remove search replicas. This will require changes to create and update services. We should also add changes to prevent requests from routing to a search replica by excluding them from a primary's replication targets in ReplicationGroup.