nvidia-gpu: add entity-manager support

The commit add support for reading of the entity-manager configurations
to the gpu dbus sensor app.

Tested.

Build an image for gb200nvl-obmc machine with the following patches
cherry picked. This patches are needed to enable the mctp stack.

https://gerrit.openbmc.org/c/openbmc/openbmc/+/79312
https://gerrit.openbmc.org/c/openbmc/openbmc/+/79422

Copy the gpusensor app and run it.
```
root@gb200nvl-obmc:~# ./nvidiagpusensor
```

The app is detecting entity-manager configuration on gb200nvl-obmc
machine. The app is also able to detect all the endpoints from the mctp
service dbus tree.

Change-Id: I05a0597964bcc0c135484fed714b6f677adc5891
Signed-off-by: Harshit Aghera <haghera@nvidia.com>
diff --git a/src/nvidia-gpu/NvidiaGpuSensor.hpp b/src/nvidia-gpu/NvidiaGpuSensor.hpp
new file mode 100644
index 0000000..14627fc
--- /dev/null
+++ b/src/nvidia-gpu/NvidiaGpuSensor.hpp
@@ -0,0 +1,123 @@
+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2024-2025 NVIDIA CORPORATION &
+ * AFFILIATES. All rights reserved. SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#include "Thresholds.hpp"
+#include "Utils.hpp"
+#include "sensor.hpp"
+
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/steady_timer.hpp>
+#include <boost/container/flat_map.hpp>
+#include <sdbusplus/asio/connection.hpp>
+#include <sdbusplus/asio/object_server.hpp>
+#include <sdbusplus/message.hpp>
+
+#include <memory>
+#include <string>
+#include <vector>
+
+constexpr const char* sensorPathPrefix = "/xyz/openbmc_project/sensors/";
+constexpr const char* sensorType = "NvidiaMctpVdm";
+
+struct GpuTempSensor :
+    public Sensor,
+    public std::enable_shared_from_this<GpuTempSensor>
+{
+  public:
+    /**
+     * @brief Constructor for GpuTempSensor
+     * @param conn D-Bus connection
+     * @param io Boost ASIO I/O context for asynchronous operations
+     * @param mctpRequester MCTP protocol requester for GPU communication
+     * @param name Name of the sensor
+     * @param sensorConfiguration Configuration string for the sensor
+     * @param objectServer D-Bus object server
+     * @param thresholdData Vector of threshold configurations
+     * @param pollRate How often to poll for new readings
+     * @param deviceInfo Information about the GPU device
+     * @param verbose Whether to enable verbose logging
+     */
+    GpuTempSensor(std::shared_ptr<sdbusplus::asio::connection>& conn,
+                  boost::asio::io_context& io, const std::string& name,
+                  const std::string& sensorConfiguration,
+                  sdbusplus::asio::object_server& objectServer,
+                  std::vector<thresholds::Threshold>&& thresholdData);
+
+    /**
+     * @brief Destructor
+     */
+    ~GpuTempSensor() override;
+
+    /**
+     * @brief Check if any thresholds have been crossed
+     * @details Overrides the base class method to implement GPU-specific
+     * threshold checking
+     */
+    void checkThresholds() override;
+
+  private:
+    /**
+     * @brief Discover available GPUs on the system
+     */
+    void discoverGpus();
+
+    /**
+     * @brief Process MCTP endpoints discovered on the system
+     *
+     * @param[in] ec Error code from the D-Bus method call
+     * @param[in] ret Object tree results containing MCTP endpoint information
+     */
+    void queryEndpoints(const boost::system::error_code& ec,
+                        const GetSubTreeType& ret);
+
+    /**
+     * @brief Process configuration properties for MCTP endpoints
+     *
+     * @param[in] ec Error code from the D-Bus properties method call
+     * @param[in] configs Map of configuration properties for the endpoint
+     */
+    void processEndpoint(const boost::system::error_code& ec,
+                         const SensorBaseConfigMap& endpoint);
+
+    /**
+     * @brief Timer for scheduling sensor reads
+     */
+    boost::asio::steady_timer waitTimer;
+
+    /**
+     * @brief D-Bus connection
+     */
+    std::shared_ptr<sdbusplus::asio::connection> conn;
+
+    /**
+     * @brief D-Bus object server
+     */
+    sdbusplus::asio::object_server& objectServer;
+};
+
+/**
+ * @brief Create GPU temperature sensors
+ * @param io Boost ASIO I/O context
+ * @param objectServer D-Bus object server
+ * @param sensors Map to store created sensors
+ * @param dbusConnection D-Bus connection
+ */
+void createSensors(
+    boost::asio::io_context& io, sdbusplus::asio::object_server& objectServer,
+    boost::container::flat_map<std::string, std::shared_ptr<GpuTempSensor>>&
+        sensors,
+    std::shared_ptr<sdbusplus::asio::connection>& dbusConnection);
+
+/**
+ * @brief Handle D-Bus interface removal events
+ * @param message D-Bus message containing interface removal information
+ * @param sensors Map of GPU temperature sensors to check for removal
+ */
+void interfaceRemoved(
+    sdbusplus::message_t& message,
+    boost::container::flat_map<std::string, std::shared_ptr<GpuTempSensor>>&
+        sensors);