TLS 1.3 post-handshake auth

[essen]

Is your feature request related to a problem? Please describe.
TLS 1.3 supports post-handshake client certificate authentication. This allows accepting connections without requiring the client's certificate and then later on requesting it for additional checks to be performed.

In OpenSSL this is done via SSL_verify_client_post_handshake and friends.

I have seen a few comments on tickets indicating it hasn't been prioritised, but none are open, so I figured I would open one to track this. Personally I would like to use it to help me test Gun's handling of TLS alerts, but I could also see it being useful to provide increased security for specific operations in a long lived connection to e.g. RabbitMQ. I'll see if I can dedicate time helping to implement this.

[IngelaAndin]

Yes we are aware that this TLS-1.3 feature is missing. So far it has not made top priority. Hopefully during OTP-29 development cycle. I can say it will most definitely not be implemented by us before OTP-28. But if anyone is really interested in having this soon they can always make a PR.