Add pin length parameter for CR-10/Ender 3 beds

[DJLuigi10]

I have an ender 3 and the hotbed screws are approximately 48mm diagonally from the bed corner. The XL pins are still way short of being useable, unless I opt to drill some holes in my hotbed, which I'm not too keen to do.

I would love to be able to adjust the length of the pins and/or depth of the underhanging portion of the bracket to accomodate as I really like this design, and would love to be able to make use of this exquisitely designed part.