title: Native Queries in ReactiveMongoRepository
body: <p>Is there a way to perform native queries in ReactiveMongoRepository like JpaRepository which uses @Query annotation? Likes this:</p>
  
  <pre><code>public interface MyRepository extends JpaRepository&lt;MyClass, Long&gt; {
  @Query(value= "SELECT id FROM my_table ORDER BY RAND() LIMIT 1" , nativeQuery = true)
  public Long getRandomData();
  }
  </code></pre>
  
  <p>This is the query I want to perform:</p>
  
  <pre><code>db.airportControl.aggregate([
    { "$match": {
      "$and": [
        { "$expr": { "$eq": [{ "$year": "$boardingDate" }, 2019] }},
        { "airportDestiny": "MAD"}
      ]
    }},
    { "$group": {
      "_id": null,
      "average": {
        "$avg": { "$subtract": ["$boardingDate", "$securityGateDate"] }
      }
    }}
  ]);
  </code></pre>