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

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

Depends on D6207

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

Reviewers: varun, tomek, jon, max, michal

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6211