[DLPack] Update to include DLPack C Exchange API

This PR updates to include dlpack C exchange api for fast exchange
at c-extension level without going through python.

Author: tqchen

spec/draft/design_topics/data_interchange.rst

@@ -85,6 +85,18 @@ page gives a high-level specification for data exchange in Python using DLPack.
    below. They are not required to return an array object from ``from_dlpack``
    which conforms to this standard.
 
+
+DLPack C Exchange API
+---------------------
+
+DLPack 1.3 introduces a C-level exchange API that can be used to speed up
+data exchange between arrays at the C-extension level. This API is available via
+the ``type(array_instance).__dlpack_c_exchange_api__`` attribute on the array type object.
+For more details, see the `Python specification of DLPack <https://dmlc.github.io/dlpack/latest/python_spec.html>`__
+We recommend consumer libraries to start first using the python level ``__dlpack__`` first which will covers
+most cases, then start to use the C-level ``__dlpack_c_exchange_api__`` for performance critical cases.
+
+
 Non-supported use cases
 -----------------------
 

src/array_api_stubs/_draft/array_object.py

@@ -17,6 +17,18 @@
 
 
 class _array:
+    """
+    Attributes
+    ----------
+    __dlpack_c_exchange_api__: PyCapsule
+        An optional static array type attribute store in ``type(array_instance).__dlpack_c_exchange_api__``
+        that can be used to retrieve the DLPack C-API exchange API struct in DLPack 1.3 or later to speed up
+        exchange of array data at the C extension level without going through Python-level exchange.
+        See :ref:`data-interchange` section for more details.
+    """
+    # use None for placeholder
+    __dlpack_c_exchange_api__: PyCapsule = None
+
     def __init__(self: array) -> None:
         """Initialize the attributes for the array object class."""