code: public AVQuery<T> whereStartsWith(String key, String prefix) {
      conditions.whereStartsWith(key, prefix);
      return this;
    }
docs: Add a constraint for finding string values that start with a provided string. This query will
  use the backend index, so it will be fast even for large datasets.
  
  @param key The key that the string to match is stored in.
  @param prefix The substring that the value must start with.
  @return Returns the query, so you can chain this call.