feat: add async DNS resolver I/O object with Windows IOCP support

- Add resolver class with resolve() awaitable returning io_result<resolver_results>
- Add resolver_results and resolver_entry for holding DNS resolution results
- Add resolve_flags enum for controlling resolution behavior
- Implement win_iocp_resolver_service using GetAddrInfoExW for async DNS
- Require Windows 8+ SDK (_WIN32_WINNT >= 0x0602) for GetAddrInfoExW
- Add nslookup example demonstrating resolver usage

Author: vinniefalco

example/CMakeLists.txt

@@ -9,6 +9,7 @@
 
 add_subdirectory(client)
 add_subdirectory(echo-server)
+add_subdirectory(nslookup)
 
 if(WolfSSL_FOUND)
     add_subdirectory(https-client)

example/nslookup/CMakeLists.txt

@@ -0,0 +1,22 @@
+#
+# Copyright (c) 2025 Vinnie Falco (vinnie dot falco at gmail dot com)
+#
+# Distributed under the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+#
+# Official repository: https://github.com/cppalliance/corosio
+#
+
+file(GLOB_RECURSE PFILES CONFIGURE_DEPENDS *.cpp *.hpp
+    CMakeLists.txt
+    Jamfile)
+
+source_group(TREE ${CMAKE_CURRENT_SOURCE_DIR} PREFIX "" FILES ${PFILES})
+
+add_executable(corosio_example_nslookup ${PFILES})
+
+set_property(TARGET corosio_example_nslookup
+    PROPERTY FOLDER "examples")
+
+target_link_libraries(corosio_example_nslookup
+    Boost::corosio)

example/nslookup/nslookup.cpp

@@ -0,0 +1,83 @@
+//
+// Copyright (c) 2025 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/cppalliance/corosio
+//
+
+#include <boost/corosio.hpp>
+#include <boost/capy/task.hpp>
+#include <boost/capy/ex/async_run.hpp>
+
+#include <cstdlib>
+#include <iostream>
+#include <string_view>
+
+namespace corosio = boost::corosio;
+namespace capy = boost::capy;
+
+// Coroutine that performs the DNS lookup
+capy::task<void>
+do_lookup(
+    corosio::io_context& ioc,
+    std::string_view host,
+    std::string_view service)
+{
+    corosio::resolver r(ioc);
+
+    auto [ec, results] = co_await r.resolve(host, service);
+    if (ec)
+    {
+        std::cerr << "Resolve failed: " << ec.message() << "\n";
+        co_return;
+    }
+
+    std::cout << "Results for " << host;
+    if (!service.empty())
+        std::cout << ":" << service;
+    std::cout << "\n";
+
+    for (auto const& entry : results)
+    {
+        auto ep = entry.get_endpoint();
+        if (ep.is_v4())
+        {
+            std::cout << "  IPv4: " << ep.v4_address().to_string()
+                      << ":" << ep.port() << "\n";
+        }
+        else
+        {
+            std::cout << "  IPv6: " << ep.v6_address().to_string()
+                      << ":" << ep.port() << "\n";
+        }
+    }
+
+    std::cout << "\nTotal: " << results.size() << " addresses\n";
+}
+
+int
+main(int argc, char* argv[])
+{
+    if (argc < 2 || argc > 3)
+    {
+        std::cerr <<
+            "Usage: nslookup <hostname> [service]\n"
+            "Examples:\n"
+            "    nslookup www.google.com\n"
+            "    nslookup www.google.com https\n"
+            "    nslookup localhost 8080\n";
+        return EXIT_FAILURE;
+    }
+
+    std::string_view host = argv[1];
+    std::string_view service = (argc == 3) ? argv[2] : "";
+
+    corosio::io_context ioc;
+    capy::async_run(ioc.get_executor())(
+        do_lookup(ioc, host, service));
+    ioc.run();
+
+    return EXIT_SUCCESS;
+}

include/boost/corosio.hpp

@@ -17,5 +17,7 @@
 #include <boost/corosio/consuming_buffers.hpp>
 #include <boost/corosio/read.hpp>
 #include <boost/corosio/write.hpp>
+#include <boost/corosio/resolver.hpp>
+#include <boost/corosio/resolver_results.hpp>
 
 #endif