Ensure SAI works w/ Transient Replication/Witness Replicas

Transient replication (see CASSANDRA-14404) and SAI should work together without specific integration. However, some exploratory testing is probably warranted, and there may be some details around the selection of SSTable indexes at transient replicas that may be worth inspecting. As of CASSANDRA-19018, SAI has a mechanism to separate repaired and urepaired SSTable indexes during its local queries. While repaired data should be removed from transient replicas, it still might be a good idea to avoid looking at repaired SSTables altogether if we can (and especially if the removal of repaired data lags at all, creating a window where we look at it unnecessarily).