#1760 Implement tls-exporter channel binding for TLS 1.3
Description of feature:
Implement the TLS 1.3 channel bindings currently defined in https://datatracker.ietf.org/doc/draft-ietf-kitten-tls-channel-bindings-for-tls13/ (in RFC publication queue, it is unlikely that any changes will be made before the final document is ready).
Prosody currently implements the tls-unique channel binding for SCRAM authentication, however, it suffers from a few potential problems described in the document linked above and is not defined at all for TLS 1.3.
To perform channel binding with TLS 1.3, a new mechanism is needed.
I propose that only when TLS 1.3 is in use tls-exporter becomes the channel binding method of choice. For any prior version of TLS, tls-unique can continue to be used.