Use Protobuf message replace Object and implement all hessian type about hessian2-codec

[Lynskylate]

There may be some problems when using the pb type directly. For example, the key of struct type fields only accepts string.It’s hard to avoid defining a HessianStruct.

message TypeStrcut {
  string type_name = 1;
  map <Value, Value> fields;
}

Maybe it’s a good idea to use the type we defined as an intermediate type, for users to use code generated from proto idl or jar package

Dubbo can generate js code from dubbo jar.

[zyfjeff]

@Lynskylate We do need to use protobuf IDL to define some complex Hessian types, such as Class, Map, List, etc. The primitive types provided by Protobuf do not meet the requirements.