Add cmd line option to pass layer change gcode.
This is useful for lithophane auto change colors.
Cmd: --custom-gcode custom_gcode_toolchange.json
Examples of the input json file can be found in the jira.
Jira: STUDIO-4070
Change-Id: I5beb5ff7d6d81028e95013e79f955e498cd3ba30
(cherry picked from commit bed6152b95c965ea87958d94ec069d8c883a8037)