Add cast (#338)

* add cast

* note what may vary across implementations

Author: MarcoGorelli

spec/API_specification/dataframe_api/column_object.py

@@ -1068,3 +1068,13 @@ def persist(self) -> Self:
             at most once per dataframe, and as late as possible in the pipeline.
         """
         ...
+
+    def cast(self, dtype: DType) -> Self:
+        """Cast specified columns to specified dtypes.
+
+        The following is not specified and may vary across implementations:
+
+        - Cross-kind casting (e.g. integer to string, or to float)
+        - Behaviour in the case of overflows
+        """
+        ...

spec/API_specification/dataframe_api/dataframe_object.py

@@ -926,3 +926,13 @@ def persist(self) -> Self:
                 ]
         """
         ...
+
+    def cast(self, dtypes: Mapping[str, DType]) -> Self:
+        """Convert specified columns to specified dtypes.
+
+        The following is not specified and may vary across implementations:
+
+        - Cross-kind casting (e.g. integer to string, or to float)
+        - Behaviour in the case of overflows
+        """
+        ...