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.

test/functional/rpc_signer.py

@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+# Copyright (c) 2017-2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test external signer.
+
+Verify that a bitcoind node can use an external signer command.
+See also wallet_signer.py for tests that require wallet context.
+"""
+import os
+import platform
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    assert_raises_rpc_error,
+)
+
+
+class RPCSignerTest(BitcoinTestFramework):
+    def mock_signer_path(self):
+        path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'mocks', 'signer.py')
+        if platform.system() == "Windows":
+            return "py " + path
+        else:
+            return path
+
+    def set_test_params(self):
+        self.num_nodes = 4
+
+        self.extra_args = [
+            [],
+            [f"-signer={self.mock_signer_path()}", '-keypool=10'],
+            [f"-signer={self.mock_signer_path()}", '-keypool=10'],
+            ["-signer=fake.py"],
+        ]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_external_signer()
+
+    def set_mock_result(self, node, res):
+        with open(os.path.join(node.cwd, "mock_result"), "w", encoding="utf8") as f:
+            f.write(res)
+
+    def clear_mock_result(self, node):
+        os.remove(os.path.join(node.cwd, "mock_result"))
+
+    def run_test(self):
+        self.log.debug(f"-signer={self.mock_signer_path()}")
+
+        assert_raises_rpc_error(-1, 'Error: restart bitcoind with -signer=<cmd>',
+            self.nodes[0].enumeratesigners
+        )
+
+        # Handle script missing:
+        assert_raises_rpc_error(-1, 'execve failed: No such file or directory',
+            self.nodes[3].enumeratesigners
+        )
+
+        # Handle error thrown by script
+        self.set_mock_result(self.nodes[1], "2")
+        assert_raises_rpc_error(-1, 'RunCommandParseJSON error',
+            self.nodes[1].enumeratesigners
+        )
+        self.clear_mock_result(self.nodes[1])
+
+        self.set_mock_result(self.nodes[1], '0 [{"type": "trezor", "model": "trezor_t", "error": "fingerprint not found"}]')
+        assert_raises_rpc_error(-1, 'fingerprint not found',
+            self.nodes[1].enumeratesigners
+        )
+        self.clear_mock_result(self.nodes[1])
+
+        result = self.nodes[1].enumeratesigners()
+        assert_equal(len(result['signers']), 2)
+        assert_equal(result['signers'][0]["fingerprint"], "00000001")
+        assert_equal(result['signers'][0]["name"], "trezor_t")
+
+if __name__ == '__main__':
+    RPCSignerTest().main()