[services][blob] Use S3 abstraction in service

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 replaces direct AWS S3 usages with the new S3Client abstraction. Also modified the MultiPartUploadSession not to be created directly, but only through S3Client.

Test Plan: Service builds. Blob service integration tests pass.

Reviewers: varun, jon, tomek

Reviewed By: varun, jon, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5787