Add a cli option for producer encoding

[denisacostaq]

Description

What

Be able to configure the encoder for the producer side
Options: model or base64
--producer_encoding Two possible values are accepted: model - if you want to send data encoded in a json mapping the model; or base64 if you want a base64 encoding. (env $PRODUCER_ENCODING) (default "model")

Why

To be able to use kafka-connect to forward data directly from kafka to elasticsearch using:
'value.converter' = 'org.apache.kafka.connect.json.JsonConverter' is required to have the messages encoded in json in the topic, base64 messages does not work.

Anything, in particular, you'd like to highlight to reviewers

This depends on Financial-Times/message-queue-go-producer#10

Would appreciate a second pair of eyes on the test
I am not quite sure how this bit works
Is there a better library for doing x

Scope and particulars of this PR (Please tick all that apply)

• Tech hygiene (dependency updating & other tech debt)
• Bug fix
• Feature
• Documentation
• Breaking change
• Minor change (e.g. fixing a typo, adding config)

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[coveralls]

Coverage decreased (-3.5%) to 31.921% when pulling ec02b38 on forward_data_using_a_model_based_encoding into 0ef127a on master.