Split client and server code

[rwb27]

I think it would be helpful to split the labthings_fastapi package into client and server parts. This avoids some confusion with optional dependencies and makes it clear which code can only run on a server.

There are a few things to figure out before we can do this, perhaps the first one is how we manage two strongly linked packages. I'd favour one repo with both packages, but that will take a bit of thought to make sure CI works.

[rwb27]

On discussion, this is probably not necessary, and creates a lot of headaches. The additional dependencies (fastapi, starlette) are unlikely to be a problem, so we will keep an eye on making sure client and server code is clear and modular, but we won't split the module for now. We will, however, abolish the server extra and install fastapi as a hard dependency, to cut down on confusion. That's done in #123.