Add U2F (FIDO U2F-over-HID) as a Scapy contrib protocol layer

[Copilot]

This PR adds a new contrib implementation for the U2F protocol, focused on U2F HID framing and core command payload parsing. It enables Scapy to build/dissect U2F initialization, continuation, error/lock flows, and MSG-carried U2F APDU payloads.

• New contrib layer: scapy.contrib.u2f

  • Introduces U2F HID packet model with frame-type dispatch:
    • U2FHID (dispatcher)
    • U2FHIDInit (init frame)
    • U2FHIDCont (continuation frame)
  • Adds U2F HID command/error enums for readable dissections.

• Command-specific payload decoding

  • Implements payload packets for core HID commands:
    • U2FHIDInitReq (nonce request)
    • U2FHIDInitResp (channel/version/capability response)
    • U2FHIDLock
    • U2FHIDError
  • Adds command-aware payload guessing from U2FHIDInit (INIT, LOCK, ERROR, MSG).

• U2F message payload support

  • Adds U2FAPDU for CMD_MSG content:
    • cla, ins, p1, p2, lc, and length-bound data
  • Supports common U2F instructions (REGISTER, AUTHENTICATE, VERSION).

• Contrib campaign coverage

  • Adds test/contrib/u2f.uts with focused build/dissect roundtrip checks for init request/response, error frame, APDU message frame, and continuation frame parsing.

load_contrib("u2f")

pkt = U2FHIDInit(cid=0xFFFFFFFF, cmd="INIT") / U2FHIDInitReq(
    nonce=hex_bytes("0102030405060708")
)
assert raw(pkt) == hex_bytes("ffffffff8600080102030405060708")

msg = U2FHIDInit(cid=0x01020304, cmd="MSG") / U2FAPDU(
    ins="AUTHENTICATE", p1=0x03, lc=2, data=hex_bytes("aabb")
)