FR: Add time offset to account for print_start

[crashmaxx]

Since klipper estimator can't expand macros like print_start, but at least kinematicly they should always take the same amount of time. I suggest an option to add a fixed number of seconds to the estimate to account for macros. This could either be a command line option or possibly a gcode comment.

This would be most useful for post processing accuracy.

It would still be up to the user to measure the time offset they need, but they wouldn't need to change it unless they changed their print_start.

I'm not aware of a command for klipper to save or output the time elapsed since the beginning of a print. But if it exists, you could add that command to the end of your print_start and automatically capture the offset needed for the user to later give to klipper estimator.