Merge #15382: util: add RunCommandParseJSON

31cf68a3ad [util] add RunCommandParseJSON (Sjors Provoost)
c17f54ee53 [ci] use boost::process (Sjors Provoost)
32128ba682 [doc] include Doxygen comments for HAVE_BOOST_PROCESS (Sjors Provoost)
3c84d85f7d [build] msvc: add boost::process (Sjors Provoost)
c47e4bbf0b [build] make boost-process opt-in (Sjors Provoost)
929cda5470 configure: add ax_boost_process (Sjors Provoost)
8314c23d7b [depends] boost: patch unused variable in boost_process (Sjors Provoost)

Pull request description:

  Prerequisite for external signer support in #16546. Big picture overview in [this gist](https://gist.github.com/Sjors/29d06728c685e6182828c1ce9b74483d).

  This adds a new dependency [boost process](https://github.com/boostorg/process/tree/boost-1.64.0). This is part of Boost since 1.64 which is part of `depends`. Because the minimum Boost version is 1.47, this functionality is skipped for older versions of Boost.

  Use `./configure --with-boost-process` to opt in, which checks for the presence of Boost::Process.

  We add `UniValue runCommandParseJSON(const std::string& strCommand)` to `system.{h,cpp}` which calls an arbitrary command and processes the JSON returned by it. This is currently only called by the test suite.

  ~For testing purposes this adds a new regtest-only RPC method `runcommand`, as well as `test/mocks/command.py` used by functional tests.~ (this is no longer the case)

  TODO:
  - [ ] review boost process in #15440

ACKs for top commit:
  achow101:
    ACK 31cf68a3ad
  hebasto:
    re-ACK 31cf68a3ad, only rebased (verified with `git range-diff`) and removed an unintentional tab character since the [previous](https://github.com/bitcoin/bitcoin/pull/15382#pullrequestreview-458371035) review.
  meshcollider:
    Very light utACK 31cf68a3ad, although I am not very confident with build stuff.
  promag:
    Code review ACK 31cf68a3ad, don't mind the nit.
  ryanofsky:
    Code review ACK 31cf68a3ad. I left some comments below that could be ignored or followed up later. The current change is clean and comprehensive.

Tree-SHA512: c506e747014b263606e1f538ed4624a8ad7bcf4e025cb700c12cc5739964e254dc04a2bbb848996b170e2ccec3fbfa4fe9e2b3976b191222cfb82fc3e6ab182d

test/lint/lint-includes.sh

@@ -63,6 +63,7 @@ EXPECTED_BOOST_INCLUDES=(
     boost/optional.hpp
     boost/preprocessor/cat.hpp
     boost/preprocessor/stringize.hpp
+    boost/process.hpp
     boost/signals2/connection.hpp
     boost/signals2/optional_last_value.hpp
     boost/signals2/signal.hpp