Add support for ACME TLS-ALPN-01 challenge type

[aaribaud]

What would you like to be added

The only challenge type supported by the step CLI is HTTP-01.
Add support for TLS-ALPN-01.

Why this is needed

HTTP-01 requires the host to open port 80 for ACME traffic in addition to port 443 for normal traffic
TLS-ALPN-01 support would allow routing all traffic trough port 443 alone.

[aaribaud]

Actual use cases would certainly have a web server already bound to port 443, so I assume the following:

• web server would be configured once to route ACME ALPN to some local port (e.g. 443 on 127.0.1.0, or 4443)
• step CLI would start its own TLS ACME responder on-demand on said local port (like it does for HTTP)

[aaribaud]

Also: willing to work on implementing this :)

[dopey]

Hey @aaribaud 👋 . We'd love a contribution here! Let us know if you'd like a walkthrough of any sections of the code, or if you've got any questions.