FastAPI should not be a core dependency

[ml-evs]

Currently FastAPI is a core dependency as it is required to generate the OpenAPI schemas. We should relegate it to a server dependency only, so we also avoid having to pull starlette etc. which can have awkward conflicting versions for those just wanting to use the models from this package.

I don't necessarily want to rush this through, but I think it would be a good idea to remove in time for 0.18, even if it breaks some existing packages.

[ml-evs]

As #1199 shows, there is some minor interaction between the schemas, validator, and the FastAPI exceptions class that should be untangled before dropping the dep. Ideally we could extract all of the validator code out of the server module to make this clearer.

Let's leave this until a future release.