Revert "Removing Pyarrow dependency (#7146)" (#7209)

This reverts commit 2116fd3bca.

python/ray/includes/buffer.pxi

@@ -10,8 +10,8 @@ cdef class Buffer:
     """Cython wrapper class of C++ `ray::Buffer`.
 
     This class implements the Python 'buffer protocol', which allows
-    us to use it for calls into Python libraries without having to
-    copy the data.
+    us to use it for calls into pyarrow (and other Python libraries
+    down the line) without having to copy the data.
 
     See https://docs.python.org/3/c-api/buffer.html for details.
     """

python/ray/includes/libcoreworker.pxd

@@ -45,9 +45,6 @@ ctypedef void (*ray_callback_function) \
     (shared_ptr[CRayObject] result_object,
      CObjectID object_id, void* user_data)
 
-ctypedef void (*plasma_callback_function) \
-    (CObjectID object_id, int64_t data_size, int64_t metadata_size)
-
 cdef extern from "ray/core_worker/profiling.h" nogil:
     cdef cppclass CProfiler "ray::worker::Profiler":
         void Start()
@@ -196,5 +193,3 @@ cdef extern from "ray/core_worker/core_worker.h" nogil:
         CRayStatus SetResource(const c_string &resource_name,
                                const double capacity,
                                const CClientID &client_Id)
-
-        void SubscribeToAsyncPlasma(plasma_callback_function callback)