add basic webserver that can do most of what i need

- serve static files, serving index.html or optionally listings for directories
- redirects
- reverse-proxy, forwarding requests to a backend

these are configurable through the config file. a domain and path regexp have to
be configured. path prefixes can be stripped.  configured domains are added to
the autotls allowlist, so acme automatically fetches certificates for them.

all webserver requests now have (access) logging, metrics, rate limiting.
on http errors, the error message prints an encrypted cid for relating with log files.

this also adds a new mechanism for example config files.

http/mtasts.go

@@ -13,7 +13,9 @@ import (
 )
 
 func mtastsPolicyHandle(w http.ResponseWriter, r *http.Request) {
-	log := xlog.WithCid(mox.Cid())
+	log := func() *mlog.Log {
+		return xlog.WithContext(r.Context())
+	}
 
 	host := strings.ToLower(r.Host)
 	if !strings.HasPrefix(host, "mta-sts.") {
@@ -28,7 +30,7 @@ func mtastsPolicyHandle(w http.ResponseWriter, r *http.Request) {
 	}
 	domain, err := dns.ParseDomain(host)
 	if err != nil {
-		log.Errorx("mtasts policy request: bad domain", err, mlog.Field("host", host))
+		log().Errorx("mtasts policy request: bad domain", err, mlog.Field("host", host))
 		http.NotFound(w, r)
 		return
 	}
@@ -49,7 +51,7 @@ func mtastsPolicyHandle(w http.ResponseWriter, r *http.Request) {
 		}
 		d, err := dns.ParseDomain(s)
 		if err != nil {
-			log.Errorx("bad domain in mtasts config", err, mlog.Field("domain", s))
+			log().Errorx("bad domain in mtasts config", err, mlog.Field("domain", s))
 			http.Error(w, "500 - internal server error - invalid domain in configuration", http.StatusInternalServerError)
 			return
 		}