Add zilla context to MQTT consumer groups

[vordimous]

Describe the desired outcome from the user's perspective
As an MQTT broker admin, I want to deploy multiple MQTT brokers where some may use the same underlying kafka topics. I want to control which broker instances create the same consumer group IDs for MQTT clients with static client IDs.

Acceptance criteria

• Add zilla context to MQTT consumer group IDs, ex namespace binding_name
• Running multiple Zillas with different contexts creates "separated" MQTT brokers while connected to the same Kafka topics
• Add MQTT consumer group explaination zilla-docs#113

Additional context
This could be implemented in the same way gRPC consumer group ID naming is for consistency.