Add support for ECH

[gaardiolor]

Hello,

It would be nice if support for Encrypted ClientHello (ECH) could be added. It's becoming mainstream, most browsers support it now.

It replaces ESNI:

• https://datatracker.ietf.org/doc/draft-ietf-tls-esni/
• https://www.ietf.org/archive/id/draft-ietf-tls-esni-22.txt (although the document is still named esni, the title contents changed to Encrypted ClientHello)
• https://blog.cloudflare.com/encrypted-client-hello/

Perhaps #345 can then be closed.

[tomato42]

yes, would be nice to have, but I'm working on post-quantum algorithms now, won't be picking it up any time soon