ADXL345 Prusa Mk3S/+ Extruder support

I wanted to try using input shaper with Klipper on my Prusa MK3S, so I needed to mount the ADXL345 accelerometer on the extruder head. I designed a mount that allows you to install it behind the fan.

BOM:
- x2 M3 threaded inserts (4mm long and diameter of 5mm)
- x2 M3x5 screws
- all the original screws and nut 

Printing instructions:
Print it flat, without supports. I used Prusament PETG with a 0.15 mm layer height, 100% infill, and 3 perimeters.

I added a way to mount the ADXL345 accelerometer to the part cooling fan bracket.