[services][blob] Introduce S3 primitives

Summary:
Introduced two S3 primitives:

• S3 path - a convenience struct to store object and bucket name. See C++ counterpart
• Multipart upload session - needed for Put requests to upload large blobs to S3. See C++ counterpart. Its implementation is added in the subsequent diff.

Depends on D5678

Test Plan: Added two simple unit tests for S3Path.

Reviewers: jon, varun, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5679