[ODM] Add support for meta fields

[dmba]

First of all thanks for your work on ODM package!

Is it possible to add some meta fields that can be set via ODM client e.g. createdAt/updatedAt/id. Something like:

@JsonSerializable()
class Post {
   @id
   final String id:
   @createdAt
   final Timestamp createdAt;
   @updatedAt
   final Timestamp createdAt;
}