replace dapr with kafka

I've switched to Kafka. A few notes:

• from the package name, it seems that we'd like this to be general. but the effort of having this is probably not worth it. as such, while we abstracted some parts, the underlying Kafka implementation will still be visible on things like the fake. We could probably try to create an interface over things, but again, I don't think it's worth it at the moment.
• the Kafka package we are using is not allowing sending the configuration as params, so we needed to replace the values in the config at runtime. we only did "brokers" as the others didn't seem important enough. but if we need any of them, we can probably expose them.
• because we want to use the fake, we can't bind into the container a Kafka "client" (similar to what we had with Dapr). again, probably not an issue, but worth mentioning.