Create initial FastAPI Cookiecutter Boilerplate

Create Cookiecutter boilerplate for FastAPI API, with sensible defaults allowing:

• Tox testing through CI
• External settings loading
• Development all through familiar compose.sh scripts

Spun out from https://gitlab.developers.cam.ac.uk/uis/devops/iam/deploy-identity/-/issues/27 in order to create this as a reuse-able cookiecutter template