Make `requests` an optional dependency

[EnriqueSoria]

Why feature request?

We don't want to force users of python-api-consumers to stick to requests library. But we want to include a ready to use implementation because requests is the defacto library for consuming HTTP endpoints.

What is needed to do?

• Mark requests as an optional dependency (extras)
• Maybe raise a custom exception if someone tries to import something from src/api_consumer/requests.py and doesn't have requests installed.