[services][blob] Move current implementation to cpp directory

Summary:
Until the Rust implementation is complete, the current C++ implementation
can live next to it.

• Moved the C++ Blob Service code to the services/blob/cpp directory
• Adjusted Dockerfile and CMakeLists.txt to use the new location

Test Plan:
Docker image builds, the following commands are executing successfully:

• yarn run-blob-service
• yarn run-integration-tests blob

Reviewers: varun, jon, tomek, marcin

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5675