probe: Add support for configuring lift_speed

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

config/example-extras.cfg

@@ -569,6 +569,10 @@
 #sample_retract_dist: 2.0
 #   The distance (in mm) to lift the toolhead between each sample (if
 #   sampling more than once). The default is 2mm.
+#lift_speed: 5.0
+#   Speed (in mm/s) of the Z axis when lifting the probe between
+#   samples. The default is to use the same value as the 'speed'
+#   parameter.
 #samples_result: average
 #   The calculation method when sampling more than once - either
 #   "median" or "average". The default is average.