ipc: Expose an RPC interface over the -ipcbind socket

This allows `bitcoin-cli` to connect to the node via IPC instead TCP to execute
RPC methods in an upcoming commit.

src/interfaces/init.h

@@ -9,6 +9,7 @@
 #include <interfaces/echo.h>
 #include <interfaces/mining.h>
 #include <interfaces/node.h>
+#include <interfaces/rpc.h>
 #include <interfaces/wallet.h>
 
 #include <memory>
@@ -36,6 +37,7 @@ public:
     virtual std::unique_ptr<Mining> makeMining() { return nullptr; }
     virtual std::unique_ptr<WalletLoader> makeWalletLoader(Chain& chain) { return nullptr; }
     virtual std::unique_ptr<Echo> makeEcho() { return nullptr; }
+    virtual std::unique_ptr<Rpc> makeRpc() { return nullptr; }
     virtual Ipc* ipc() { return nullptr; }
     virtual bool canListenIpc() { return false; }
     virtual const char* exeName() { return nullptr; }