rproxy, the proxy in front of each app, asks servewaf and fails open (closes #171)

`smallwebwaf rproxy` reads the first 64 KiB of each body, asks servewaf over
HTTP/1.1, and passes what servewaf allows to the app within its own size and
time limits. It reports each request servewaf answered. SWRP_DEFAULT_DISPOSITION
decides when servewaf gives no answer. A request it cannot put to servewaf as
sent, it refuses itself.

Duplication: internal/rproxy copies passing-on code from internal/proxy until
#172 removes the originals.
Rule suppressed: gosec G704 and G705, on servewaf's URL and the visitor's answer.
Rule suppressed: paralleltest, on the test of a switch rproxy refuses.
Judgement call: the SWRP_ size and time limits take off; SWRP_TIMEOUT_MS does not.
Judgement call: every target not starting with one / gets 400.

Model: opus-5-5

internal/requestlog/requestlog.go

@@ -1,6 +1,7 @@
 // Package requestlog writes the lines smallwebwaf prints on stdout: one
-// JSON object per request, or two from servewaf, marked "type":"request", and
-// the process's own messages as JSON lines marked "type":"process".
+// JSON object per request, two from servewaf and a short one from rproxy, marked
+// "type":"request", and the process's own messages as JSON lines marked
+// "type":"process".
 package requestlog
 
 import (
@@ -193,10 +194,50 @@ type Line struct {
 	DurationUpstreamTotal     *float64 `json:"duration_upstream_total,omitempty"`
 }
 
+// RProxyLine is rproxy's short line in the request log about one request. The
+// field names, and their order, are those of docs/request-log.md. A field that
+// does not apply to a request is left out of its line.
+//
+//nolint:tagliatelle // docs/request-log.md names the fields in snake_case
+type RProxyLine struct {
+	Type          string `json:"type"`
+	Time          string `json:"time"`
+	RequestID     string `json:"request_id"`
+	ClientIP      string `json:"client_ip"`
+	Host          string `json:"host"`
+	Method        string `json:"method"`
+	Path          string `json:"path"`
+	Query         string `json:"query"`
+	Status        int    `json:"status"`
+	RequestBytes  int64  `json:"request_bytes"`
+	ResponseBytes int64  `json:"response_bytes"`
+	// DurationRequest runs in milliseconds from the request's headers being read
+	// until the app has been sent the whole request, or the answer has started
+	// if that is sooner. DurationResponse runs from then until the answer ends.
+	DurationRequest  float64 `json:"duration_request"`
+	DurationResponse float64 `json:"duration_response"`
+	// Disposition is servewaf's answer or, while DefaultDisposition is true, the
+	// value of SWRP_DEFAULT_DISPOSITION. It is "" when servewaf was not asked.
+	Disposition        string `json:"disposition,omitempty"`
+	DefaultDisposition bool   `json:"default_disposition,omitempty"`
+}
+
 // Write writes line to w as one JSON line marked "type":"request".
 func Write(w io.Writer, line *Line) error {
 	line.Type = "request"
 
+	return writeJSONLine(w, line)
+}
+
+// WriteRProxyLine writes line to w as one JSON line marked "type":"request".
+func WriteRProxyLine(w io.Writer, line *RProxyLine) error {
+	line.Type = "request"
+
+	return writeJSONLine(w, line)
+}
+
+// writeJSONLine writes line to w as one JSON line.
+func writeJSONLine(w io.Writer, line any) error {
 	encoded, err := json.Marshal(line)
 	if err != nil {
 		return fmt.Errorf("encode the request log line: %w", err)