multiprocess: Add echoipc RPC method and test

Add simple interfaces::Echo IPC interface with one method that just takes and
returns a string, to test multiprocess framework and provide an example of how
it can be used to spawn and call between processes.

src/init/bitcoin-node.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <interfaces/echo.h>
 #include <interfaces/init.h>
 #include <interfaces/ipc.h>
 #include <node/context.h>
@@ -21,6 +22,7 @@ public:
     {
         m_node.init = this;
     }
+    std::unique_ptr<interfaces::Echo> makeEcho() override { return interfaces::MakeEcho(); }
     interfaces::Ipc* ipc() override { return m_ipc.get(); }
     NodeContext& m_node;
     std::unique_ptr<interfaces::Ipc> m_ipc;