[SNAPSHOT] Add BWC layer to .si / segments_N hashing

Due to additional safety added in #7351 we compute now a strong hash for
.si and segments_N files which are compared during snapshot / restore.
Old snapshots don't have this hash which can cause unnecessary copying
of large amount of data. This commit adds the ability to fetch this
hash from the blob store if needed.

Closes #7434
[SNAPSHOT] Ensure BWC layer can read chunked blobs
Use empty BytesRef if we read from <= 1.3.3
[TEST] only bump replicas if we have enough nodes in the cluster
[TEST] only expand to 1 replica in SnapshotBackwardsCompatibilityTest
Use physical name to compare files from snapshot metadata

The comparison and read code in the BlobStoreIndexShardRepository
used the physicalName and Name in reverse order. This caused
SnapshotBackwardsCompatibilityTest to fail.