STUN NOMINATION attribute code should not be `0xC001`

[ibc]

Context

STUN NOMINATION attribute is not standard. It's used by libwebrtc (and then others) and uses code 0xC001 (in mediasoup too).

Problem

However the value 0xC001 has already been assigned by IANA to ENF-FLOW-DESCRIPTION (see https://www.iana.org/assignments/stun-parameters/stun-parameters.xml), so this means conflict.

More info

• Issue reported in libwebrtc: https://issues.webrtc.org/issues/496629058 (we must monitor it).
• For example, Pion uses 0x0030, which is currently unused.