Create explain_dr.c and move DestReceiver-related code there.

explain.c has grown rather large, and the code that deals with the
DestReceiver that supports the SERIALIZE option is pretty easily severable
from the rest of explain.c; hence, move it to a separate file.

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: http://postgr.es/m/CA+TgmoYutMw1Jgo8BWUmB3TqnOhsEAJiYO=rOQufF4gPLWmkLQ@mail.gmail.com

src/backend/commands/meson.build

@@ -22,6 +22,7 @@ backend_sources += files(
   'dropcmds.c',
   'event_trigger.c',
   'explain.c',
+  'explain_dr.c',
   'explain_format.c',
   'extension.c',
   'foreigncmds.c',