improvement(cleanup): remove backend from argus client

Some cleanup work had to be done to better structure argus client for less problematic management:

Removed old db stuff, that cause confusion when searching for db/models related things. Along with old migration and prod dump scripts.
moved common modules for backend and client to common module next to backend and client modules
excluded backend module from client package. Including backend might cause import error due backend-only required packages
created unit tests for argus client package - build, install, basic import

scylladb / argus