kernel: move RunCommandParseJSON to its own file

Because libbitcoinkernel does not include this new object, this has the
side-effect of eliminating the unnecessary boost::process dependency.

src/util/run_command.cpp

@@ -0,0 +1,64 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#if defined(HAVE_CONFIG_H)
+#include <config/bitcoin-config.h>
+#endif
+
+#include <util/run_command.h>
+
+#include <tinyformat.h>
+#include <univalue.h>
+
+#ifdef ENABLE_EXTERNAL_SIGNER
+#if defined(__GNUC__)
+// Boost 1.78 requires the following workaround.
+// See: https://github.com/boostorg/process/issues/235
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnarrowing"
+#endif
+#include <boost/process.hpp>
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+#endif // ENABLE_EXTERNAL_SIGNER
+
+UniValue RunCommandParseJSON(const std::string& str_command, const std::string& str_std_in)
+{
+#ifdef ENABLE_EXTERNAL_SIGNER
+    namespace bp = boost::process;
+
+    UniValue result_json;
+    bp::opstream stdin_stream;
+    bp::ipstream stdout_stream;
+    bp::ipstream stderr_stream;
+
+    if (str_command.empty()) return UniValue::VNULL;
+
+    bp::child c(
+        str_command,
+        bp::std_out > stdout_stream,
+        bp::std_err > stderr_stream,
+        bp::std_in < stdin_stream
+    );
+    if (!str_std_in.empty()) {
+        stdin_stream << str_std_in << std::endl;
+    }
+    stdin_stream.pipe().close();
+
+    std::string result;
+    std::string error;
+    std::getline(stdout_stream, result);
+    std::getline(stderr_stream, error);
+
+    c.wait();
+    const int n_error = c.exit_code();
+    if (n_error) throw std::runtime_error(strprintf("RunCommandParseJSON error: process(%s) returned %d: %s\n", str_command, n_error, error));
+    if (!result_json.read(result)) throw std::runtime_error("Unable to parse JSON: " + result);
+
+    return result_json;
+#else
+    throw std::runtime_error("Compiled without external signing support (required for external signing).");
+#endif // ENABLE_EXTERNAL_SIGNER
+}