Move external signer out of wallet module

This commit moves the ExternalSigner class and RPC methods out of the wallet module.

The enumeratesigners RPC can be used without a wallet since #21417.
With additional modifications external signers could be used without a wallet in general, e.g. via signrawtransaction.

The signerdisplayaddress RPC is ranamed to walletdisplayaddress because it requires wallet context.
A future displayaddress RPC call without wallet context could take a descriptor argument.

This commit fixes a rpc_help.py failure when configured with --disable-wallet.

src/rpc/external_signer.cpp

@@ -0,0 +1,70 @@
+// 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 <chainparamsbase.h>
+#include <external_signer.h>
+#include <rpc/server.h>
+#include <rpc/util.h>
+#include <util/strencodings.h>
+#include <rpc/protocol.h>
+
+#ifdef ENABLE_EXTERNAL_SIGNER
+
+static RPCHelpMan enumeratesigners()
+{
+    return RPCHelpMan{
+        "enumeratesigners",
+        "Returns a list of external signers from -signer.",
+        {},
+        RPCResult{
+            RPCResult::Type::OBJ, "", "",
+            {
+                {RPCResult::Type::ARR, "signers", /* optional */ false, "",
+                {
+                    {RPCResult::Type::STR_HEX, "masterkeyfingerprint", "Master key fingerprint"},
+                    {RPCResult::Type::STR, "name", "Device name"},
+                },
+                }
+            }
+        },
+        RPCExamples{""},
+        [](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue {
+            const std::string command = gArgs.GetArg("-signer", "");
+            if (command == "") throw JSONRPCError(RPC_MISC_ERROR, "Error: restart bitcoind with -signer=<cmd>");
+            std::string chain = gArgs.GetChainName();
+            UniValue signers_res = UniValue::VARR;
+            try {
+                std::vector<ExternalSigner> signers;
+                ExternalSigner::Enumerate(command, signers, chain);
+                for (ExternalSigner signer : signers) {
+                    UniValue signer_res = UniValue::VOBJ;
+                    signer_res.pushKV("fingerprint", signer.m_fingerprint);
+                    signer_res.pushKV("name", signer.m_name);
+                    signers_res.push_back(signer_res);
+                }
+            } catch (const ExternalSignerException& e) {
+                throw JSONRPCError(RPC_MISC_ERROR, e.what());
+            }
+            UniValue result(UniValue::VOBJ);
+            result.pushKV("signers", signers_res);
+            return result;
+        }
+    };
+}
+
+void RegisterSignerRPCCommands(CRPCTable &t)
+{
+// clang-format off
+static const CRPCCommand commands[] =
+{ // category              actor (function)
+  // --------------------- ------------------------
+  { "signer",              &enumeratesigners,      },
+};
+// clang-format on
+    for (const auto& c : commands) {
+        t.appendCommand(c.name, &c);
+    }
+}
+
+#endif // ENABLE_EXTERNAL_SIGNER