Add supporting column parameter for last function(flux-dsl)

It will be great,  if `flux-dsl` will support this query:
```

from(bucket: "example-bucket")
  |> range(start: -1h)
  |> pivot(rowKey:["_time"], columnKey:["_field"], valueColumn:"_value")
  |> last(column: "_time")

  ```
  On `flux-dsl` : 
  ```
  ....
  .last("_time");
  ```