Train picker arbiter kernel

More update: now the device, PickyBoi is its own package and this MR just adds an arbiter kernel. This kernel should essentially follow similar logic to PickyBoi: mask is all ones on captured trains and all zeroes on ignored trains.

Update: see #23 (closed) for design considerations. tl;dr: add a device which can sit downstream of preview assembler*, forwarding data only for specific trains. Then add a frame selection arbiter kernel which accomplishes the same thing, but for full data stream with frame selection feature.

*The device is currently entirely generic, just because. It will set its output schema based on what it gets. Not sure if it has other use cases than this, but oh well.