tarsum: allow for alternate block ciphers

Not breaking the default cipher and algorithm for calculating checksums,
but allow for using alternate block ciphers during the checksum
calculation.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #5956 from vbatts/vbatts-tarsum_hashes

tarsum: allow for generic hashes