[ncp] add a vendor hook to notify when a frame is sent and removed (#2468)

src/ncp/ncp_base.cpp

@@ -706,6 +706,10 @@ void NcpBase::HandleFrameRemovedFromNcpBuffer(NcpFrameBuffer::FrameTag aFrameTag
 
     SuccessOrExit(SendQueuedResponses());
 
+#if OPENTHREAD_ENABLE_NCP_VENDOR_HOOK
+    VendorHandleFrameRemovedFromNcpBuffer(aFrameTag);
+#endif
+
     // Check if `HOST_POWER_STATE` property update is required.
 
     if (mHostPowerStateHeader)

src/ncp/ncp_base.hpp

@@ -293,6 +293,7 @@ protected:
 
 #if OPENTHREAD_ENABLE_NCP_VENDOR_HOOK
     otError VendorCommandHandler(uint8_t aHeader, unsigned int aCommand);
+    void VendorHandleFrameRemovedFromNcpBuffer(NcpFrameBuffer::FrameTag aFrameTag);
 #endif // OPENTHREAD_ENABLE_NCP_VENDOR_HOOK
 
     otError CommandHandler_NOOP(uint8_t aHeader);