sense/realtime

This is kind of implemented as its own API and is even less stable than [sense](..). The [asyncapi.yaml] is an untested but nearly complete description of the Sense real-time API. I would have preferred to use a code generator, similar to how I did in the internal/client, but the code generators for AsyncAPI are not mature and I couldn't get them to produce anything useful. If you're reading this from a timeline in which they are useful, consider splitting this package into an internal/realtime (or integrate it with internal/client) and possibly merge this package or a higher-level version of it with sense.