Add support to Credentials for CourseCertificate signatures

Work to deprecate the Old Mongo Modulestore is currently blocked from completion because signature image artifacts for course certificates are stored in Mongo's GridFS.

There is a vision that web certificates for courses will migrate to the Credentials IDA. This work is a small piece of that, specified to unblock the DEPR work while remaining aligned with the overall vision.

This work should consider:

• image retrieval load should be done in a way that makes sense operationally given the expect load generated by this new workload.
• signature images are uploaded in Studio today and are part of the OLx import/export process.