Support protobuf in locally defined schema

[adrdc]

Schema may be defined in a local .json file. avro, and json formats are currently supported. json will have actual field names+types inline. avro can provide a pointer to it's schema.

for example for avro it may look like this - sample.avsc has the actual schema definition

{
  "schemaName": "sensors",
  "tableName": "temperature",
  "streamName": "temperature",
  "event": [
    {
      "dataFormat": "avro",
      "dataSchema": "etc/pravega/sample.avsc"
    }
  ]
}

For protobuf, 'dataSchema' would be a pointer to the protobuf FieldDescriptor.
The only missing piece is that for protobuf we need 1 more piece of information - the fully qualified type name of the class (as a string)