Magistrala Rules engine

[drasko]

FEATURE REQUEST

1. Is there an open issue addressing this request? If it does, please add a "+1" reaction to the
   existing issue, otherwise proceed to step 2.

2. Describe the feature you are requesting, as well as the possible use case(s) for it.
   Kupier is lightweight stream RE from EMQX team. It can be a very good fit from Mainflux.

We should consider creating a NATS Source, but actually it can also be connected directly to our MQTT broker (MQTT Source already exists)

Other alternative is Grule, but Kuiper seems more adapted for our use-case.

3. Indicate the importance of this feature to you (must-have, should-have, nice-to-have).
   should-have

[drasko]

The analysis showed good fit, with open question of scalability of Kuiper. We decided to proceed with integration.

@darkodraskovic is working on this, and there is already draft PR opened: https://github.com/mainflux/mainflux/pull/1337.

Moving it to 0.13, to allow more time for proper integration.

[dborovcanin]

@drasko Since there has been no activity on this particular RE integration, and I also closed PR since it would be too complex to update, I suggest that we close this issue.

[dborovcanin]

Also, this is worth exploring https://github.com/rulego/rulego.

[dborovcanin]

We don't necessarily enforce message storage but we can process and publish back to NATS. We also need a no-code value comparison option since that's by far the most common use case and the option to create a custom script for more knowledgeable people and more complex logic.
Probably 90% of cases are comparison ones.
We process and push back to NATS under a special topic (say re/<chan_id>) so the user can choose what channel to write in DB - raw or processed by subscribing the Writer to the appropriate channel.

We also need predefined actions - like push to NATS, store to writer, webhook, and more direct integrations, such as sending an email, Slack notification, SMS, or something else.

By pushing to the channel, we can implicitly chain rules, keeping the workflow fairly simple (only three steps of in-channel->logic->action).