[Dart][Jaguar] and proto serialization support

Created by: jaumard

Here is my current work to allow the generator of jaguar dart to support either json or proto serialization. I've added a serializarion param to choose json or proto. I proto models should be generated as proto files and not dart files. For now I'm blocked because I'm trying to do this:

message {{classname}} {
    {{#vars}}
    {{{datatype}}} {{name}} = {{@index}};
    {{/vars}}
}

Apparently the {{@index}} is not supported :/ and return empty. But in proto definition you need to specify the index of each field like:

message Person {
  string name = 1;
  int32 id = 2;
  string email = 3;

}