motion_queuing: Add new module to help with motion queues and flushing

Create a new module to assist with host management of motion queues.
Register all MCU_stepper objects with this module and use the module
for step generation.

All steppers will now automatically generate steps whenever
toolhead._advance_flush_time() is invoked.  It is no longer necessary
for callers to individually call stepper.generate_steps().

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/motion_queuing.py

@@ -0,0 +1,19 @@
+# Helper code for low-level motion queuing and flushing
+#
+# Copyright (C) 2025  Kevin O'Connor <kevin@koconnor.net>
+#
+# This file may be distributed under the terms of the GNU GPLv3 license.
+import logging
+
+class PrinterMotionQueuing:
+    def __init__(self, config):
+        self.printer = config.get_printer()
+        self.steppers = []
+    def register_stepper(self, config, stepper):
+        self.steppers.append(stepper)
+    def flush_motion_queues(self, must_flush_time, max_step_gen_time):
+        for stepper in self.steppers:
+            stepper.generate_steps(max_step_gen_time)
+
+def load_config(config):
+    return PrinterMotionQueuing(config)