Add `fylgje` for BIFROST

fylgje is a visualizer to follow the raw-data being produced on an instrument by leveraging the ar51 flatbuffer messages sent as 'detector samples'.

This information is ephemeral but can be useful to alert users and interested parties about data-quality problems caused by faulty hardware.

This request adds a separate target to the CMake configuration which is a first working draft fylgje intended for BIFROST. fylgje copies implementation details from daqlite, so it seems worthwhile keeping its code close. At the moment, however, there is code duplication between the targets which may or may not be deemed acceptable. If the level of duplication is not acceptable, it would be necessary to refactor those common parts to a common location in the repository.