multiprocess: add interfaces::ExternalSigner class

Add interfaces::ExternalSigner to let signer objects be passed between
processes and signer code to run in the original process, without
multiple processes linking and running signer code.

src/interfaces/node.h

@@ -6,7 +6,6 @@
 #define BITCOIN_INTERFACES_NODE_H
 
 #include <consensus/amount.h>
-#include <external_signer.h>
 #include <net.h>        // For NodeId
 #include <net_types.h>  // For banmap_t
 #include <netaddress.h> // For Network
@@ -50,6 +49,16 @@ struct BlockAndHeaderTipInfo
     double verification_progress;
 };
 
+//! External signer interface used by the GUI.
+class ExternalSigner
+{
+public:
+    virtual ~ExternalSigner() {};
+
+    //! Get signer display name
+    virtual std::string getName() = 0;
+};
+
 //! Top-level interface for a bitcoin node (bitcoind process).
 class Node
 {
@@ -111,8 +120,8 @@ public:
     //! Disconnect node by id.
     virtual bool disconnectById(NodeId id) = 0;
 
-    //! List external signers
-    virtual std::vector<ExternalSigner> externalSigners() = 0;
+    //! Return list of external signers (attached devices which can sign transactions).
+    virtual std::vector<std::unique_ptr<ExternalSigner>> listExternalSigners() = 0;
 
     //! Get total bytes recv.
     virtual int64_t getTotalBytesRecv() = 0;