Implement base for nextgen rpc subsystem (#487)

Fixes #494

src/rpc/common/Specs.cpp

@@ -0,0 +1,42 @@
+//------------------------------------------------------------------------------
+/*
+    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.
+*/
+//==============================================================================
+
+#include <rpc/common/Specs.h>
+
+#include <boost/json/value.hpp>
+
+namespace RPCng {
+
+[[nodiscard]] MaybeError
+FieldSpec::validate(boost::json::value const& value) const
+{
+    return validator_(value);
+}
+
+[[nodiscard]] MaybeError
+RpcSpec::validate(boost::json::value const& value) const
+{
+    for (auto const& field : fields_)
+        if (auto ret = field.validate(value); not ret)
+            return Error{ret.error()};
+
+    return {};
+}
+
+}  // namespace RPCng