[services][backup] Move C++ impl to 'old' directory

Summary:
For the needs of the Rust service development, I moved the C++ implementation to the old dir.

• Modified Dockerfile accordingly
• Modified CMakeLists accordingly
• Left blob_client untouched as it is convenient for me to keep it where it is.

Test Plan:

• Docker container builds
• Service integration tests are passing

Reviewers: tomek, varun, jon

Reviewed By: tomek, jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5841