Add engine field in ElasticResultMeta

[Revitate]

I try to use meta engine on my project. But ElasticResultMeta does not have engine field.
According to https://www.elastic.co/guide/en/app-search/current/meta-engines-guide.html

I suggest to add this line.

@freezed
class ElasticResultMeta with _$ElasticResultMeta {
  factory ElasticResultMeta({
    /// The relevance of the result
    double? score,
    String? engine, // <-- add this to model
  }) = _ElasticResultMeta;

  factory ElasticResultMeta.fromJson(Map<String, dynamic> json) =>
      _$ElasticResultMetaFromJson(json);
}