Merge bitcoin/bitcoin#28981: Replace Boost.Process with cpp-subprocess

d5a715536e build: remove boost::process dependency for building external signer support (Sebastian Falbesoner)
70434b1c44 external_signer: replace boost::process with cpp-subprocess (Sebastian Falbesoner)
cc8b9875b1 Add `cpp-subprocess` header-only library (Hennadii Stepanov)

Pull request description:

  Closes https://github.com/bitcoin/bitcoin/issues/24907.

  This PR is based on **theStack**'s [work](https://github.com/bitcoin/bitcoin/issues/24907#issuecomment-1466087049).

  The `subprocess.hpp` header has been sourced from the [upstream repo](https://github.com/arun11299/cpp-subprocess) with the only modification being the removal of convenience functions, which are not utilized in our codebase.

  Windows-related changes will be addressed in subsequent follow-ups.

ACKs for top commit:
  achow101:
    reACK d5a715536e
  Sjors:
    re-tACK d5a715536e
  theStack:
    Light re-ACK d5a715536e
  fanquake:
    ACK d5a715536e - with the expectation that this code is going to be maintained as our own. Next PRs should:

Tree-SHA512: d7fb6fecc3f5792496204190afb7d85b3e207b858fb1a75efe483c05260843b81b27d14b299323bb667c990e87a07197059afea3796cf218ed8b614086bd3611

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",

test/lint/lint-spelling.py

@@ -14,7 +14,7 @@ from subprocess import check_output, STDOUT, CalledProcessError
 from lint_ignore_dirs import SHARED_EXCLUDED_SUBTREES
 
 IGNORE_WORDS_FILE = 'test/lint/spelling.ignore-words.txt'
-FILES_ARGS = ['git', 'ls-files', '--', ":(exclude)build-aux/m4/", ":(exclude)contrib/seeds/*.txt", ":(exclude)depends/", ":(exclude)doc/release-notes/", ":(exclude)src/qt/locale/", ":(exclude)src/qt/*.qrc", ":(exclude)contrib/guix/patches"]
+FILES_ARGS = ['git', 'ls-files', '--', ":(exclude)build-aux/m4/", ":(exclude)contrib/seeds/*.txt", ":(exclude)depends/", ":(exclude)doc/release-notes/", ":(exclude)src/qt/locale/", ":(exclude)src/qt/*.qrc", ":(exclude)contrib/guix/patches", ":(exclude)src/util/subprocess.hpp"]
 FILES_ARGS += [f":(exclude){dir}" for dir in SHARED_EXCLUDED_SUBTREES]