Add basic structure

This MR adds a basic structure to support new model development, e.g., codegen.

• codesuggestions.api - FastAPI routes including middlewares
• codesuggestions.auth - authentication providers, e.g., GitLab
• codesuggestions.models - repositories to access model generations, e.g., codegen or polycoder
• config.py - app configuration
• completions.py - business logic aka use case

Ref #11 (closed)