4-Add BVG API Client

[Timelessprod]

Changes:

• Add requests-mock library to requirements.txt to be able to mock API calls in tests
• Call Pytest in test script
• Add Python config file to hold some constants for different environments (test & prod)
• Add script to run app
• Add BVG API client with support for /radar and /stops endpoints
• Add tests for BVG API client including some API response sample files
• Update Docker Compose config to use existing environment variables or default to given values

[Timelessprod]

Tests are passing, approving