Add Support for TLS-ALPN-01 challenge type

[shimonelbaz]

is it possible to add support this validation protocol?
TLS-ALPN-01

[arontsang]

Support for TLS-ALPN-01 needs to be integrated with the TLS termination proxy.

In theory it would be possible to setup HA Proxy to forward the Acme-tls/1 requests to kcert.

[arontsang]

Further looking into SslStream, I am not sure that the required extension points exist to implement this.

At a minimum, we need to be able to switch Certs based on the ALPN negotiation.