[wpiutil] Add DsClient (#3654)

This is a libuv-based implementation of the Driver Station client
for getting robot IPs from the Driver Station port 1742 server.

wpiutil/examples/dsclient/dsclient.cpp

@@ -0,0 +1,35 @@
+// 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 <cstdio>
+
+#include "fmt/format.h"
+#include "wpi/DsClient.h"
+#include "wpi/EventLoopRunner.h"
+#include "wpi/Logger.h"
+#include "wpi/uv/Error.h"
+
+namespace uv = wpi::uv;
+
+static void logfunc(unsigned int level, const char* file, unsigned int line,
+                    const char* msg) {
+  std::fprintf(stderr, "(%d) %s:%d: %s\n", level, file, line, msg);
+}
+
+int main() {
+  wpi::Logger logger{logfunc, 0};
+
+  // Kick off the event loop on a separate thread
+  wpi::EventLoopRunner loop;
+  std::shared_ptr<wpi::DsClient> client;
+  loop.ExecAsync([&](uv::Loop& loop) {
+    client = wpi::DsClient::Create(loop, logger);
+    client->setIp.connect(
+        [](std::string_view ip) { fmt::print("got IP: {}\n", ip); });
+    client->clearIp.connect([] { std::fputs("cleared IP\n", stdout); });
+  });
+
+  // wait for a keypress to terminate
+  std::getchar();
+}