Add interface to PBJ records

[lpetrovic05]

A PBJ record has a static PROTOBUF variable for encoding/decoding a record to/from protobuf. In order to create a tool that can encode an arbitrary PBJ record, it would be useful to introduce an interface to these records (PbjRecord). The interface would have a method like getProtoCodec that would return this static variable. The tool could then encode an arbitrary record without any additional data.