Add configurability to the server

Rather than hard-coding the Things that make up the server, this MR introduces a configuration file as defined in LabThings#73. I've also added generic Camera and Stage things, as a first stab at making it possible to use different hardware. I've added an OpenCV camera class (currently lacking any settings), as a first step towards a useful alternative to Picamera2.

This MR includes an updated readme so it's clear how to run the server, and also will need updates to the disk image - in particular the service that runs the server.

The necessary changes are in LabThings as of v0.0.4 and I've pinned the dependency to v0.0.6 as there are breaking changes coming as we work towards more stable version of labthings-fastapi.