Add QueueSize epics_signal_rw to NDPluginBaseIO

[mrakitin]

diff --git a/src/ophyd_async/epics/adcore/_core_io.py b/src/ophyd_async/epics/adcore/_core_io.py
index 796857911..15de202b1 100644
--- a/src/ophyd_async/epics/adcore/_core_io.py
+++ b/src/ophyd_async/epics/adcore/_core_io.py
@@ -38,6 +38,7 @@ class NDPluginBaseIO(NDArrayBaseIO):
         self.nd_array_address = epics_signal_rw_rbv(int, prefix + "NDArrayAddress")
         self.array_size0 = epics_signal_r(int, prefix + "ArraySize0_RBV")
         self.array_size1 = epics_signal_r(int, prefix + "ArraySize1_RBV")
+        self.queue_size = epics_signal_rw(int, prefix + "QueueSize")
         super().__init__(prefix, name)