Support verification of s3 delivered files

We need a method to verify that the files that have been delivered and stored in s3 have not been modified. This should include issues with a bad actor modifying the files in the bucket.
It would also be nice if the verification could take place even without access to the D4S2 database.

We have discussed adding a MD5 check sum to the metadata and/or adding a digital signature.
There is a chicken and egg problem with creating a digital signature based on the metadata, and storing that signature in the metadata.