fix: specify column to search for FTS (#1572)

Before this we ignored the `fts_columns` parameter, and for now we
support to search on only one column, it could lead to an error if we
have multiple indexed columns for FTS

---------

Signed-off-by: BubbleCal <bubble-cal@outlook.com>

python/src/index.rs

@@ -98,6 +98,13 @@ impl Index {
             inner: Mutex::new(Some(LanceDbIndex::LabelList(Default::default()))),
         })
     }
+
+    #[staticmethod]
+    pub fn fts() -> PyResult<Self> {
+        Ok(Self {
+            inner: Mutex::new(Some(LanceDbIndex::FTS(Default::default()))),
+        })
+    }
 }
 
 #[pyclass(get_all)]