chore: Cache ibis columns property (#2563)

feat: Cache ibis columns property

Author: FBruzzesi

narwhals/_ibis/dataframe.py

@@ -65,6 +65,7 @@ def __init__(
         self._version = version
         self._backend_version = backend_version
         self._cached_schema: dict[str, DType] | None = None
+        self._cached_columns: list[str] | None = None
         validate_backend_version(self._implementation, self._backend_version)
 
     @staticmethod
@@ -213,7 +214,13 @@ def schema(self) -> dict[str, DType]:
 
     @property
     def columns(self) -> list[str]:
-        return list(self.native.columns)
+        if self._cached_columns is None:
+            self._cached_columns = (
+                list(self.schema)
+                if self._cached_schema is not None
+                else list(self.native.columns)
+            )
+        return self._cached_columns
 
     def to_pandas(self) -> pd.DataFrame:
         # only if version is v1, keep around for backcompat