Replace environment variables in matchers as appropriate

[avirut]

I'd like to be able to use environment variables within my l4 matchers, e.g.:

"match": [
    {
        "tls": {
            "sni": [
                "{env.DOMAIN}"
            ]
        }
    },
    {
        "http": [
            {
                "host": [
                    "{env.DOMAIN}"
                ]
            }
        ]
    }
],

Unfortunately, it doesn't seem like l4 uses the replacer to do this yet. Would be great if this was possible!

[mholt]

This should be pretty straightforward, I think. Anyone want to submit a patch?

[vnxme]

@tls_sni_env tls sni {env.DOMAIN} is now possible after caddyserver/caddy#6480 is merged.