cmd/tailscale/cli, ipn/ipnlocal: [funnel] add stream mode

Adds ability to start Funnel in the foreground and stream incoming
connections. When foreground process is stopped, Funnel is turned
back off for the port.

Exampe usage:
```
TAILSCALE_FUNNEL_V2=on tailscale funnel 8080
```

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/localapi/localapi.go

@@ -99,6 +99,7 @@ var handler = map[string]localAPIHandler{
 	"set-expiry-sooner":           (*Handler).serveSetExpirySooner,
 	"start":                       (*Handler).serveStart,
 	"status":                      (*Handler).serveStatus,
+	"stream-serve":                (*Handler).serveStreamServe,
 	"tka/init":                    (*Handler).serveTKAInit,
 	"tka/log":                     (*Handler).serveTKALog,
 	"tka/modify":                  (*Handler).serveTKAModify,
@@ -857,6 +858,31 @@ func (h *Handler) serveServeConfig(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// serveStreamServe handles foreground serve and funnel streams. This is
+// currently in development per https://github.com/tailscale/tailscale/issues/8489
+func (h *Handler) serveStreamServe(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitWrite {
+		// Write permission required because we modify the ServeConfig.
+		http.Error(w, "serve stream denied", http.StatusForbidden)
+		return
+	}
+	if r.Method != "POST" {
+		http.Error(w, "POST required", http.StatusMethodNotAllowed)
+		return
+	}
+	var req ipn.ServeStreamRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeErrorJSON(w, fmt.Errorf("decoding HostPort: %w", err))
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	if err := h.b.StreamServe(r.Context(), w, req); err != nil {
+		writeErrorJSON(w, fmt.Errorf("streaming serve: %w", err))
+		return
+	}
+	w.WriteHeader(http.StatusOK)
+}
+
 func (h *Handler) serveCheckIPForwarding(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitRead {
 		http.Error(w, "IP forwarding check access denied", http.StatusForbidden)