Fully isolate server code from other submodules

[ml-evs]

As part of #1199 I've been tidying up which other modules use server code (and thus require a dependency on FastAPI).

There's 3 bits that remain:

• optimade.server.warnings can be moved out to the top-level package, leaving an import shim in its place for backwards compat
• optimade.server.exceptions can be reimplemented without inheriting from FastAPIHTTPException, which will mean that exceptions still work when emitted from client code (e.g., the filter parsing that happens on the client side).
• optimade.server.schemas is trickier; it is currently used by the validator as well as the main server. The validator is only using bits that required pydantic, but I don't see a huge value in splitting up the optimade.server.schemas submodule, so in Demote FastAPI to a server dep only #1199 I've just put an import guard in. In the future we could consider moving this submodule to the top-level entirely.