Add footprint for Mini Circuits FG873
Datasheet: https://ww2.minicircuits.com/case_style/FG873.pdf
This is the footprint for https://github.com/KiCad/kicad-packages3D/pull/365
I am not sure the DFN_QFN lib is the right one for this part. I think we might want to add some library that holds specialized packages that just don't fit anywhere else.