webrtc: accept iceServers: [] provided in POST /webrtc, support trickle ICE

Thanks https://github.com/ayufan/camera-streamer/pull/65

util/http/json.hh

@@ -0,0 +1,27 @@
+#pragma once
+
+extern "C" {
+#include "http.h"
+}
+
+#include <stdio.h>
+#include <nlohmann/json.hpp>
+
+inline nlohmann::json http_parse_json_body(http_worker_t *worker, FILE *stream, unsigned max_body_size)
+{
+  std::string text;
+
+  size_t i = 0;
+  size_t n = (size_t)worker->content_length;
+  if (n < 0 || n > max_body_size)
+    n = max_body_size;
+
+  text.resize(n);
+
+  while (i < n && !feof(stream)) {
+    i += fread(&text[i], 1, n-i, stream);
+  }
+  text.resize(i);
+
+  return nlohmann::json::parse(text);
+}