Provide coroutine process interface for handler (#521)

Fixes #522

unittests/rpc/handlers/impl/FakesAndMocks.h

@@ -98,6 +98,37 @@ public:
     }
 };
 
+// example handler
+class CoroutineHandlerFake
+{
+public:
+    using Input = TestInput;
+    using Output = TestOutput;
+    using Result = RPCng::HandlerReturnType<Output>;
+
+    RPCng::RpcSpecConstRef
+    spec() const
+    {
+        using namespace RPCng::validation;
+
+        // clang-format off
+        static const RPCng::RpcSpec rpcSpec = {
+            {"hello", Required{}, Type<std::string>{}, EqualTo{"world"}},
+            {"limit", Type<uint32_t>{}, Between<uint32_t>{0, 100}} // optional field
+        };
+        // clang-format on
+
+        return rpcSpec;
+    }
+
+    Result
+    process(Input input, boost::asio::yield_context* ptrYield) const
+    {
+        return Output{
+            input.hello + '_' + std::to_string(input.limit.value_or(0))};
+    }
+};
+
 class NoInputHandlerFake
 {
 public: