Add authentication support for MAST-U data

[khalsz]

In future we will need to support MAST-U data, which will be in embargo. We need to have

a location where we can store embargoed data
an authentication system which can support giving access to both UKAEA users and external collaborators.
There is some middleware we might be able to use:
https://fastapi-keycloak-middleware.readthedocs.io/en/latest/index.html

Keycloak can also be used from a docker image, which may be useful for testing locally:
https://www.keycloak.org/getting-started/getting-started-docker