add `truncated` field to /first-rows
On the Hub's side, we have to guess if the result has been truncated. For example, if the result has 35 rows: maybe it has not been truncated because the split only had 35 rows, or maybe it has been truncated because the cells contain heavy data -> *we cannot detect*.

It would be more explicit to return `truncated: boolean` in the response.
add `truncated` field to /first-rows: On the Hub's side, we have to guess if the result has been truncated. For example, if the result has 35 rows: maybe it has not been truncated because the split only had 35 rows, or maybe it has been truncated because the cells contain heavy data -> *we cannot detect*.

It would be more explicit to return `truncated: boolean` in the response.