conncheck: NOMINATION STUN attribute support

Attribute is proposed here: https://tools.ietf.org/html/draft-thatcher-ice-renomination-00
WebRTC supports this attribute: controlling side provides attribute value increased by one
each time selected pair is changed:
https://chromium.googlesource.com/external/webrtc/+/3c7d599750405bc734e1d5adbf1b54265b725a9d/p2p/base/p2ptransportchannel.cc#1821
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build-and-test

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