[feature] arm attachment bracket

[alexrusselldev]

we need a bracket for attaching things to the arm

we have the landing gear bracket, it might make sense at this point to split these out into a bracket base module that takes parameters to define length, wall thickness, etc, and then specialised brackets built on top of this base

we could implement an endstop boolean parameter that drives whether the bracket should also act as a cap for the end of the arms

i will also open / update issues for brackets for a variety of applications:

• landing gear
• motor pods
• ESC mounts
• wiring tidys