[backup] Extract constants to lib

Summary:
Extracted constants that are not specific to backup service to lib.
The DynamoDB constants is needed in other services as well.
The gRPC constants aren't used anymore in the backup service, but they can be needed elsewhere.

Test Plan: cargo check

Reviewers: michal, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8936