Support `MapType` for `ElementAt`

[comphead]

in Apache Spark, the ElementAt supports Map, and Comet gates for Array only, we should support Map

  override def inputTypes: Seq[AbstractDataType] = {
    (left.dataType, right.dataType) match {
      case (arr: ArrayType, e2: IntegralType) if (e2 != LongType) =>
        Seq(arr, IntegerType)
      case (MapType(keyType, valueType, hasNull), e2) =>
        TypeCoercion.findTightestCommonType(keyType, e2) match {
          case Some(dt) => Seq(MapType(dt, valueType, hasNull), dt)
          case _ => Seq.empty
        }
      case (l, r) => Seq.empty

    }
  }

Originally posted by @comphead in #4677