[services][backup] CreateBackup 1/3 - create handler module

Summary:

• Created module structure for gRPC service handlers to keep it organized:

- mod service (existing grpc service mod)
  - mod handlers (groups all handler submodules)
    - create_backup
    - add_attachments
    - ...

• Scaffolded the CreateBackupHandler structure, containing the whole endpoint logic. This is 1:1 analogy with Blob service PutHandler
• Implemented usage of this structure in the endpoint handler function.

The CreateBackupHandler consists of two phases

1. Non-data mode - processing non-chunk inputs like device_id, user_id etc.
2. Data mode - processing backup data chunks
3. Finish - postprocessing, saving to db etc.

Depends on D6181

Test Plan: This does nothing yet, subsequent diffs will add logic to this code.

Reviewers: varun, tomek, jon, max

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6196