wallet: add -signer argument for external signer command

Create basic ExternalSigner class with contructor. A Signer(<cmd>)
is added to CWallet on load if -signer=<cmd> is set.

src/wallet/external_signer.cpp

@@ -0,0 +1,8 @@
+// Copyright (c) 2018-2021 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 <wallet/external_signer.h>
+#include <util/system.h>
+
+ExternalSigner::ExternalSigner(const std::string& command, const std::string& fingerprint): m_command(command), m_fingerprint(fingerprint) {}