mod_saslauth: Implement RFC 9266 'tls-exporter' channel binding (#1760)
Brings back SCRAM-SHA-*-PLUS from its hiatus brought on by the earlier
channel binding method being undefined for TLS 1.3, and the increasing
deployment of TLS 1.3.
See 1bfd238e05ad and #1542
Requires future version of LuaSec, once support for this key material
export method is merged.
See https://github.com/brunoos/luasec/pull/187