feat: Graceful shutdown (#1801)

Fixes #442.

src/app/Stopper.cpp

@@ -0,0 +1,52 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "app/Stopper.hpp"
+
+#include <boost/asio/spawn.hpp>
+
+#include <functional>
+#include <thread>
+#include <utility>
+
+namespace app {
+
+Stopper::~Stopper()
+{
+    if (worker_.joinable())
+        worker_.join();
+}
+
+void
+Stopper::setOnStop(std::function<void(boost::asio::yield_context)> cb)
+{
+    boost::asio::spawn(ctx_, std::move(cb));
+}
+
+void
+Stopper::stop()
+{
+    // Do nothing if worker_ is already running
+    if (worker_.joinable())
+        return;
+
+    worker_ = std::thread{[this]() { ctx_.run(); }};
+}
+
+}  // namespace app