Add option to set the offsets via a custom macro

[irrenhaus]

This PR adds a new config variable offset_macro.

When set to the name of a gcode_macro, instead of setting the calculated Z offset via SET_GCODE_OFFSET the configured macro is called with a parameter Z being set to the calculated offset value.

The reasoning behind this lies in toolchanging systems, in my case specifically Lineux: https://github.com/Bikin-Creative/Lineux-Toolchanger

For compensating any differences of the different tools in XYZ, all tools are measured against a common reference tool, most often T0.
Upon a toolchange to a different tool then SET_GCODE_OFFSET is used to adjust the motion system to the different nozzle position of the tool.
This in turn invalidates any changes made by klipper_z_calibration.

This PR now allows to:

• Measure all tools of the toolchanger against T0
• On each print, run CALIBRATE_Z for T0 calling a custom macro that lets the toolchanging code know about the new offset
• Have the toolchanging code apply the measure offset additionally to the measured tool offsets at each toolchange

Example macro for Lineux:

[gcode_macro Z_CALIBRATION_SET_OFFSET_MACRO]
gcode:
  {% set z_offset = params.Z|default(0.0)|float %}
  RESPOND MSG="Forwarding Z offset adjustment of {z_offset} to Lineux"
  SET_GCODE_VARIABLE MACRO=_btc_Variables VARIABLE=gcode_offset_z_adjust VALUE={z_offset}
  SET_GCODE_OFFSET Z_ADJUST={z_offset}

Lineux then runs
SET_GCODE_OFFSET Z_ADJUST={printer["gcode_macro _btc_Variables"].gcode_offset_z_adjust} MOVE=1 on each toolchange.