Add support for schema_registry* settings on output plugin

[fbaligand]

Currently, avro schema registry can be defined on "kafka" input to read Kafka messages that are encoded with an avro schema.
I'm very interested to have the same feature, but on "kafka" output, so that we can send Kafka messages are encoded with an avro schema defined in an avro schema registry.

I currently need this to send messages that respect an avro schema defined in a schema registry.