Make `requests` an optional dependency

## Why feature request?

We don't want to force users of `python-api-consumers` to stick to [`requests`](https://requests.readthedocs.io/en/latest/) 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)](https://python-poetry.org/docs/pyproject/#extras)
- Maybe raise a custom exception if someone tries to import something from `src/api_consumer/requests.py` and doesn't have `requests` installed.