Properly handle S3 digests

[imsdu]

Motivation
The S3 checksum changes depending if a file was upload via a single operation or a multipart one.
This should be reflected in the digest saved in the database

Exemple of a checksum returned by S3 for the multipart case:

kFsM2p15+Jbp2K0FIF0y1zIWlEJOt5052qlU8IRQPtM=-3

Acceptance criteria

1. The checksum and the number of parts is correctly parsed by Nexus for both cases
2. Both the chcksum and the number of parts is saved
3. The number of parts is also indexed in BG/ES

Technical details
A new case class is added to the Digest ADT