Support for JSON-encoded Avro

Avro supports two serialization encoding methods: binary and JSON

cf. https://avro.apache.org/docs/1.11.1/specification/#encodings 

flink-avro currently has a hard-coded assumption that Avro data is binary-encoded (and cannot process Avro data that has been JSON-encoded).

I propose adding a new optional format option to flink-avro: avro.encoding

It will support two options: 'binary' and 'json'. 

It unset, it will default to 'binary' to maintain compatibility/consistency with current behaviour.