`BaseAPIClient`: add `request_session` constructor argument

[risicle]

What problem does the pull request solve?

This allows a pre-constructed requests.Session object to be used by the api client, which is useful for some esoteric use cases.

Checklist

• [x] I’ve used the pull request template
• [ ] I’ve written unit tests for these changes
• [x] I’ve updated the documentation in
  • [ ] notifications-tech-docs repository
  • [x] CHANGELOG.md
• [x] I’ve bumped the version number in
  • [x] notifications_python_client/__init__.py
• [ ] I've added new environment variables to
  • [ ] notifications-python-client/scripts/generate_docker_env.sh
  • [ ] notifications-python-client/tox.ini
  • [ ] CONTRIBUTING.md

[quis]

This could do with a lightweight test in https://github.com/alphagov/notifications-python-client/blob/main/tests/notifications_python_client/test_base_api_client.py to validate that multiple instances of the client are sharing the same object