FaceDancer: Implement Initial Design Specification / RFP

[ktemkin]

Create an initial design document describing a FaceDancer backend for LUNA.

This should:

• [x] Include plans for the core backend.
• [ ] Accommodate future FPGA acceleration, where possible.
• [ ] Accommodate fancy USBProxy enhancements.

[antoinevg]

Latest version can be found in:

https://github.com/antoinevg/GSG/blob/main/luna/design/rfp.md