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/interfaces/init.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <interfaces/chain.h>
+#include <interfaces/echo.h>
 #include <interfaces/init.h>
 #include <interfaces/node.h>
 #include <interfaces/wallet.h>
@@ -11,5 +12,6 @@ namespace interfaces {
 std::unique_ptr<Node> Init::makeNode() { return {}; }
 std::unique_ptr<Chain> Init::makeChain() { return {}; }
 std::unique_ptr<WalletClient> Init::makeWalletClient(Chain& chain) { return {}; }
+std::unique_ptr<Echo> Init::makeEcho() { return {}; }
 Ipc* Init::ipc() { return nullptr; }
 } // namespace interfaces