Add option to use snake_case names by for serialization by default

[jakemac53]

I think this should live on the JsonSerializable annotation, and would apply to all fields under the annotated object.

Example usage:

@JsonSerializable(snakeCaseFields: true)
class Thing {
  final Thing nestedThing;
}

Would be equivalent to the following:

@JsonSerializable()
class Thing {
  @JsonKey(name: 'nested_thing')
  final Thing nestedThing;
}

Enums are a bit of an edge case, its not clear if it should apply automatically to enum values or not. One specific edge case here is if an enum value has its own explicit JsonKey annotation, once that is supported. Would that always override this setting?

[kevmoo]

Not planning to do this now that you can wire up JsonValue on enum values.