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/echo.cpp

@@ -0,0 +1,18 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// 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 <memory>
+
+namespace interfaces {
+namespace {
+class EchoImpl : public Echo
+{
+public:
+    std::string echo(const std::string& echo) override { return echo; }
+};
+} // namespace
+std::unique_ptr<Echo> MakeEcho() { return std::make_unique<EchoImpl>(); }
+} // namespace interfaces

src/interfaces/echo.h

@@ -0,0 +1,26 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_INTERFACES_ECHO_H
+#define BITCOIN_INTERFACES_ECHO_H
+
+#include <memory>
+#include <string>
+
+namespace interfaces {
+//! Simple string echoing interface for testing.
+class Echo
+{
+public:
+    virtual ~Echo() {}
+
+    //! Echo provided string.
+    virtual std::string echo(const std::string& echo) = 0;
+};
+
+//! Return implementation of Echo interface.
+std::unique_ptr<Echo> MakeEcho();
+} // namespace interfaces
+
+#endif // BITCOIN_INTERFACES_ECHO_H

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

src/interfaces/init.h

@@ -11,6 +11,7 @@ struct NodeContext;
 
 namespace interfaces {
 class Chain;
+class Echo;
 class Ipc;
 class Node;
 class WalletClient;
@@ -29,6 +30,7 @@ public:
     virtual std::unique_ptr<Node> makeNode();
     virtual std::unique_ptr<Chain> makeChain();
     virtual std::unique_ptr<WalletClient> makeWalletClient(Chain& chain);
+    virtual std::unique_ptr<Echo> makeEcho();
     virtual Ipc* ipc();
 };