Add support to Credentials for CourseCertificate signatures

[e0d]

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.

[e0d]

The PR for the proposed design is here

• [FC-0004] API to manage certificate signatures #1837