Rename headers to .hpp (#1154)

Fixes #1153

src/web/interface/Concepts.hpp

@@ -0,0 +1,44 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2023, 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.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "web/interface/ConnectionBase.hpp"
+
+#include <boost/beast.hpp>
+#include <boost/beast/core/error.hpp>
+
+#include <memory>
+#include <string>
+
+namespace web {
+
+/**
+ * @brief Specifies the requirements a Webserver handler must fulfill.
+ */
+template <typename T>
+concept SomeServerHandler =
+    requires(T handler, std::string req, std::shared_ptr<ConnectionBase> ws, boost::beast::error_code ec) {
+        // the callback when server receives a request
+        {
+            handler(req, ws)
+        };
+    };
+
+}  // namespace web