Country allow and deny lists, looked up through GeoJS (closes #44)

SWWAF_DENIED_COUNTRIES and SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES refuse a
request with 403 before its body is read or rate-limited, logged as
country_denied. internal/lookup asks GeoJS only while a list is set, 200
clients per request, one at a time, keeping answers 7 days. Failures, a
redirect or an answer leaving an address out included, are logged without
addresses; GeoJS is then left alone a second, doubling to five minutes.
Private, loopback and link-local clients are never sent.

Deviation, per the issue: no SWWAF_LOOKUP_SOURCE or SWWAF_LOOKUP_TIMEOUT; 403, not SWWAF_BAN_RESPONSE.
Deviation: GeoJS's country endpoint, not geo.json.
Judgement call: an IPv6 /64 is asked about by its first address; at most 10,000 clients wait.
Judgement call: config.go lists the ISO 3166-1 codes; no widely used library holds them.

Model: opus-5-5

internal/proxy/proxy_test.go

@@ -49,6 +49,8 @@ const (
 	responseMaxBytes        = "SWWAF_RESPONSE_MAX_BYTES"
 	trustedProxies          = "SWWAF_TRUSTED_PROXIES"
 	rateLimitPerMinute      = "SWWAF_RATE_LIMIT_PER_MINUTE"
+	deniedCountries         = "SWWAF_DENIED_COUNTRIES"
+	allowedCountries        = "SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES"
 )
 
 // output collects what smallwebwaf writes on stdout.
@@ -163,6 +165,16 @@ func startApp(t *testing.T, app http.HandlerFunc) *httptest.Server {
 func startProxy(t *testing.T, appURL string, env map[string]string) (string, *output) {
 	t.Helper()
 
+	return startProxyWithGeoJS(t, appURL, "", env)
+}
+
+// startProxyWithGeoJS is startProxy with clients' countries looked up at
+// geojsURL.
+func startProxyWithGeoJS(
+	t *testing.T, appURL, geojsURL string, env map[string]string,
+) (string, *output) {
+	t.Helper()
+
 	settings := map[string]string{"SWWAF_UPSTREAM_URL": appURL}
 	maps.Copy(settings, env)
 
@@ -180,6 +192,7 @@ func startProxy(t *testing.T, appURL string, env map[string]string) (string, *ou
 		Config:     cfg,
 		RequestLog: out,
 		ProcessLog: requestlog.NewProcessLogger(out),
+		GeoJSURL:   geojsURL,
 	})
 
 	listener, err := (&net.ListenConfig{}).Listen(t.Context(), "tcp", localhost+":0")