[services][blob] Implement S3Client methods
Summary:
Follow up https://linear.app/comm/issue/ENG-2341/blob-service-create-s3-client-abstraction which follows up https://phab.comm.dev/D5682?id=18592#inline-38028
This diff implements methods in the S3Client struct. It's not yet used anywhere.
Test Plan: Project should still compile. These functions will be tested in the next diff.
Reviewers: varun, jon, tomek
Reviewed By: varun, jon, tomek
Subscribers: ashoat, atul
Differential Revision: https://phab.comm.dev/D5786