[services][blob] Introduce S3 Client abstraction

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 introduces new struct S3Client which is a analogous to DynamoDB DatabaseClient. It's not yet used anywhere.

Test Plan: Project should still compile.

Reviewers: varun, jon, tomek

Reviewed By: varun, jon, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5785