Authorize submissions via S3 prefix write IAM policies

[xbrianh]

Submission authorization scheme:

• An IAM identity will be assigned to each submitter (user)
• Each submission will be assigned a submission-id minted by the HPP
• Each identity will be given S3 write access to the prefixes corresponding to their submission ids

Will a similar scheme work for GCP?

[xbrianh]

Possibly create utility methods to:

• Create a submission user
• Mint submission-id and authorize users