Provide context based producer variants

[mensfeld]

This PR introduces producer context variants that allow you to have different settings per topic with one producer. This allows for utilization of the same producer TCP connections while still being able to alter the dispatch requirements.

producer = WaterDrop::Producer.new do |config|
  config.kafka = {
    'bootstrap.servers': 'localhost:9092',
    'acks': '2'
  }
end

# You can define variants
low_importance = producer.with(topic_config: { acks: 1 })
hig_importance = producer.with(topic_config: { acks: 2 })

# and use them as regular producers:
low_importance.produce_async(topic: 'events', payload: event.to_json)

[nijikon]

LGTM