[noise] NoiseLink customization

[Alexey Ermishkin]

To continue our discussion on the negotiation and handshake data format, I'd
like to think about how do one add something of his own without disturbing
others.
As an example, we'll need to transmit static key's signature and signer's
data (certificate or ID). Which will happen inside  handshake messages
together with transmitting those static keys.
Should we then bind all field numbers before, say, 1000, for predefined
NoiseLink fields and allow customized ones starting with 1001 ?