Fix ZLP bug on CONTROL IN transfers

[antoinevg]

Cynthion requires the length field of any CONTROL IN transfer request in order to negotiate the transfer acknowledgement successfully.

This PR extends the Facedancer backend API as well as the core implementation to:

• Provide a new USBDevice method: control_send(self, endpoint_number: int, in_request: USBControlRequest, data: bytes, *, blocking: bool = False)
• Provide a new backend method: send_on_control_endpoint(self, endpoint_number: int, in_request: USBControlRequest, data: bytes, blocking: bool=True)
• A backwards compatible default implementation for Facedancer boards that do not need this method which:
  • Will use the length field to truncate transfers to the requested length, fixing a long-standing bug.
  • Allows the USBControlRequest to be modified by filters in USB Proxy or custom control handlers in Facedancer devices.

Depends on: greatscottgadgets/cynthion#183

---

Closes #90