Handling conversion of JSON with standard Avro encoding

[rprzystasz]

Added avro/json payload content-type and applied standard JSON->AVRO conversion mechanism for messages with this type. (based on https://avro.apache.org/docs/1.8.1/spec.html#json_encoding)

This addresses the issue with custom JSON converter (allegro/json-avro-converter#14), where more complex Union types could not be properly matched, since plain JSON does not carry type information.

For given schema:

{
    "namespace": "pl.allegro",
    "type": "record",
    "name": "User",
    "fields": [
        {
              "name": "__metadata",
              "type": ["null", {"type": "map", "values": "string"}],
              "default": null
        },
        {"name": "name", "type": "string"},
        {"name": "age",  "type": "int"},
        {"name": "favoriteColor", "type": ["null", "string"]}
    ]
}

proper avro/json formatted message is:

{"__metadata":null,"name":"Bob","age":30,"favoriteColor":{"string":"black"}}

NOTE: when namespace is defined, for custom defined types (records, etc. but not primitives) it must be prepended to type discriminator { "<namespace>.<type name>": <value> }

compared to standard application/json:

{"name":"Bob","age":30,"favoriteColor":"black"}