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/external_signer.cpp

@@ -5,8 +5,8 @@
 #include <chainparams.h>
 #include <core_io.h>
 #include <psbt.h>
+#include <util/run_command.h>
 #include <util/strencodings.h>
-#include <util/system.h>
 #include <external_signer.h>
 
 #include <algorithm>