A SERVICE_CHECKS item is now either a `str` (and treated as a REST endpoint) or
A SERVICE_CHECKS item can now be either a str
(in which case it is treated as a REST endpoint) or a callable
(and is called and the result is the check's status).
partially addresses https://gitlab.developers.cam.ac.uk/uis/devops/research-dashboard/datamart-api/-/issues/40