Add mechanism to configure gunicorn

Description

Add gunicorn configuration tooling to library.

Example manual MR here (and in django boilerplate) https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/card-api/-/merge_requests/271

Acceptance criteria

• gunicorn also uses LOG_LEVEL and/or EXTERNAL_SETTING_LOG_LEVEL
• minimal or no wiring (e.g. from ucam_observe.gunicorn import logconfig_dict gunicorn.conf.py)
• gunicorn logs are json based, confirmed via docker image or GCP