external_signer: replace boost::process with cpp-subprocess

This primarily affects the `RunCommandParseJSON` utility function.

test/lint/lint-includes.py

@@ -30,7 +30,6 @@ EXPECTED_BOOST_INCLUDES = ["boost/date_time/posix_time/posix_time.hpp",
                            "boost/multi_index/tag.hpp",
                            "boost/multi_index_container.hpp",
                            "boost/operators.hpp",
-                           "boost/process.hpp",
                            "boost/signals2/connection.hpp",
                            "boost/signals2/optional_last_value.hpp",
                            "boost/signals2/signal.hpp",