TLS-ALPN-01 Challenge support for ACME

[t-burch]

None of the big name ACME implementations support this TLS based challenge type. (NGiNX, Certbot, Apache, etc.) So it would be a nice bonus for Membrane as one of the only ACME capable gateways/reverse-proxies with this challenge type.

From Let's Encrypt docs:

This challenge is not suitable for most people. It is best suited to authors of TLS-terminating reverse proxies that want to perform host-based validation like HTTP-01, but want to do it entirely at the TLS layer in order to separate concerns. Right now that mainly means large hosting providers, but mainstream web servers like Apache and Nginx could someday implement this (and Caddy already does).

[t-burch]

TLDR; It's like the HTTP Acme challenge but instead you can do it over HTTPS so you don't have to expose a HTTP port