Proxy-based Raw Gadget tests

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Proxying USB devices proves to be an effective way to test Raw Gadget features.

A few ideas for testing Raw Gadget via proxying:

• Provide a simple in-repo proxy that supports all of the features Raw Gadget currently exposes. This proxy can evolve alongside with the new Raw Gadget features added to the dev branch. External proxies can rely on this implementation as a guideline or directly reuse it.

• Implement automatic virtual testing of Raw Gadget based on proxying the existing functions of the composite framework via Dummy HCD/UDC and USB/IP.

• Document instructions for manual testing of Raw Gadget with various physical devices, hardware UDCs, and both Linux and Windows hosts.

Also see the TODOs in the existing Raw Gadget testing tooling.