Let an instrument build its own daemon from camerad_server

[mikelangmayr]

• The server sources and their whole link interface were baked into the core executable, so a project consuming the core could not build a daemon of its own without replicating that link list
• Moves them into a camerad_server library, which the core executable now links, leaving its behaviour unchanged
• The core no longer builds its own camerad when composed as a subproject, where it only collided with the consumer target of the same name
• Adds a reference instrument showing what an instrument repository owns: an interface class, the factory translation unit, and a CMakeLists that fetches the core and links camerad_server
• CI builds that reference against the working tree, so a change that breaks a consumer fails here rather than in every instrument later
• Verified by running the consumer-built daemon against the emulator, which reports its own instrument rather than the core default

[astronomerdave]

The direction here is largely where I'm going, having an instrument build its own daemon against the core. I'd prefer not to merge #46, though, because it overlaps work that's designed and I've already done, I just didn't get it pushed before I left for a couple of days, and parts of it differ from decisions taken for this part of the design.

[mikelangmayr]

@astronomerdave that's fine. I need this for hispec and lris2, let me know when your change is going in