Dicovery mechanism

[ahilloffis]

For our application, we want to dynamically connect different devices that are represented by mango agents.
We tested a discovery mechanism via MQTT Container, which works quite well but an internal solution similar to something like Simple Service Discovery Protocol or similar for MQTT communication would be quite nice.

[rcschrg]

Just so you know, we currently have no plans to implement any version of agent discovery, but we generally think it can be a valuable feature for mango-python. @ahilloffis, if you want to come up with some sort of agent discovery, go for it and create PR.