controller_fan: Add "stepper" config option (#4447)

Allows contoller_fan sections to monitor only certain steppers instead of
all of them, similar to how heaters are currently handled.

Signed-off-by: Sophie Hirn <sophie.hirn@wyvernscale.com>

klippy/extras/stepper_enable.py

@@ -126,6 +126,8 @@ class PrinterStepperEnable:
         if name not in self.enable_lines:
             raise self.printer.config_error("Unknown stepper '%s'" % (name,))
         return self.enable_lines[name]
+    def get_steppers(self):
+        return [e.stepper for e in self.enable_lines.values()]
 
 def load_config(config):
     return PrinterStepperEnable(config)