[services][backup] AddAttachments 1/4 - create handler module

Summary:
Created module for AddAttachments endpoint and added its invocation in the grpc handler.

Depends on D6203

Test Plan: Service builds and starts. This code has no logic yet.

Reviewers: varun, tomek, jon, max

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6204