Implementation of [KIP-174](https://cwiki.apache.org/confluence/display/KAFKA/KIP-174+-+Deprecate+and+remove+internal+converter+configs+in+WorkerConfig)
Configuration properties 'internal.key.converter' and 'internal.value.converter'
are deprecated, and default to org.apache.kafka.connect.json.JsonConverter.
Warnings are logged if values are specified for either, or if properties that
appear to configure instances of internal converters (i.e., ones prefixed with
either 'internal.key.converter.' or 'internal.value.converter.') are given.
The property 'schemas.enable' is also defaulted to false for internal
JsonConverter instances (both for keys and values) if it isn't specified.
Documentation and code have also been updated with deprecation notices and
annotations, respectively.
Unit tests have been updated in `PluginsTest` to account for the new defaults for `schemas.enable` for internal key/value converters, and to ensure that (for the time being), internal key/value converters are still configurable despite being deprecated.
Author: Chris Egerton <chrise@confluent.io>
Author: Ewen Cheslack-Postava <me@ewencp.org>
Reviewers: Randall Hauch <rhauch@gmail.com>, Ewen Cheslack-Postava <ewen@confluent.io>
Closes#4693 from C0urante/kafka-5540
Chris Egerton
Konstantine Karantasis
Liquan Pei
Ewen Cheslack-Postava