Support secondary reads in ephemeralForTest

ephemeralForTest needs to implement the kLastApplied ReadSource to support secondary reads.

This will most likely require creating a SnapshotManager that records updates to lastApplied from the replication code, similar to the approach in WiredTigerRecoveryUnit.

Additionally, create a StorageEngine helper to replace this use of supportsReadConcernSnapshot in AGCFR. Something like "supportsSecondaryReads" or "supportsReadSourceLastApplied".

Another option would be to rewrite this SnapshotManager code to be above the storage engine so that any storage engine that supports timestamp reads can automatically support lastApplied reads (any maybe majority?)