Set avro/json schema on use.latest.version=True

[ifnesi]

The current AvroSerializer (src/confluent_kafka/schema_registry/avro.py) and JSONSerializer (src/confluent_kafka/schema_registry/json_schema.py) classes do not override the schema when use.latest.schema is set to True, it currently only returns the schema_id. This PR is to address that.

Please notice the same behaviour happens with ProtobufSerializer (src/confluent_kafka/schema_registry/protobuf.py), however not sure how use.latest.schema could work with Protobuf schemas as it needs to be "compiled" and the SR stores the "uncompiled" version of it. Maybe we should drop that configuration parameter out?

[cla-assistant[bot]]

All committers have signed the CLA.

[pranavrth]

@rayokota - You need to sign the CLA for the PR to get merged.