[ntcore] Add API to get and set user data on NT_Topic (#8962)

Theres many uses where we need to attach data to a topic, and doing it
this way can make it really easy and efficient.

ntcore/src/test/native/cpp/TopicTest.cpp

@@ -0,0 +1,60 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "wpi/nt/Topic.hpp"
+
+#include <gtest/gtest.h>
+
+#include "wpi/nt/NetworkTableInstance.hpp"
+
+class TopicTest : public ::testing::Test {
+ public:
+  TopicTest() : m_inst{wpi::nt::NetworkTableInstance::Create()} {}
+  ~TopicTest() override { wpi::nt::NetworkTableInstance::Destroy(m_inst); }
+
+ protected:
+  wpi::nt::NetworkTableInstance m_inst;
+};
+
+TEST_F(TopicTest, UserDataDefaultsToNull) {
+  auto topic = m_inst.GetTopic("foo");
+
+  EXPECT_EQ(nullptr, topic.GetUserData());
+}
+
+TEST_F(TopicTest, UserDataRoundTrip) {
+  auto topic = m_inst.GetTopic("foo");
+  int data = 5;
+
+  topic.SetUserData(&data);
+
+  EXPECT_EQ(&data, topic.GetUserData());
+}
+
+TEST_F(TopicTest, UserDataCanBeReplacedAndCleared) {
+  auto topic = m_inst.GetTopic("foo");
+  auto sameTopic = m_inst.GetTopic("foo");
+  int data1 = 5;
+  int data2 = 10;
+
+  topic.SetUserData(&data1);
+  EXPECT_EQ(&data1, sameTopic.GetUserData());
+
+  sameTopic.SetUserData(&data2);
+  EXPECT_EQ(&data2, topic.GetUserData());
+
+  topic.SetUserData(nullptr);
+  EXPECT_EQ(nullptr, sameTopic.GetUserData());
+}
+
+TEST_F(TopicTest, UserDataInvalidTopic) {
+  wpi::nt::Topic topic;
+  int data = 5;
+
+  EXPECT_EQ(nullptr, topic.GetUserData());
+
+  topic.SetUserData(&data);
+
+  EXPECT_EQ(nullptr, topic.GetUserData());
+}