/api/v1/fromradio: add OPTIONS handler for CORS.

This avoids hitting the 404 Not Found handler, which breaks connection
keep-alive, so this change fixes a big performance regression for Web Client in
Chrome: #5385

Tested on Heltec V3.

src/mesh/http/ContentHandler.cpp

@@ -74,6 +74,7 @@ void registerHandlers(HTTPServer *insecureServer, HTTPSServer *secureServer)
 
     ResourceNode *nodeAPIv1ToRadioOptions = new ResourceNode("/api/v1/toradio", "OPTIONS", &handleAPIv1ToRadio);
     ResourceNode *nodeAPIv1ToRadio = new ResourceNode("/api/v1/toradio", "PUT", &handleAPIv1ToRadio);
+    ResourceNode *nodeAPIv1FromRadioOptions = new ResourceNode("/api/v1/fromradio", "OPTIONS", &handleAPIv1FromRadio);
     ResourceNode *nodeAPIv1FromRadio = new ResourceNode("/api/v1/fromradio", "GET", &handleAPIv1FromRadio);
 
     //    ResourceNode *nodeHotspotApple = new ResourceNode("/hotspot-detect.html", "GET", &handleHotspot);
@@ -100,6 +101,7 @@ void registerHandlers(HTTPServer *insecureServer, HTTPSServer *secureServer)
     // Secure nodes
     secureServer->registerNode(nodeAPIv1ToRadioOptions);
     secureServer->registerNode(nodeAPIv1ToRadio);
+    secureServer->registerNode(nodeAPIv1FromRadioOptions);
     secureServer->registerNode(nodeAPIv1FromRadio);
     //    secureServer->registerNode(nodeHotspotApple);
     //    secureServer->registerNode(nodeHotspotAndroid);
@@ -121,6 +123,7 @@ void registerHandlers(HTTPServer *insecureServer, HTTPSServer *secureServer)
     // Insecure nodes
     insecureServer->registerNode(nodeAPIv1ToRadioOptions);
     insecureServer->registerNode(nodeAPIv1ToRadio);
+    insecureServer->registerNode(nodeAPIv1FromRadioOptions);
     insecureServer->registerNode(nodeAPIv1FromRadio);
     //    insecureServer->registerNode(nodeHotspotApple);
     //    insecureServer->registerNode(nodeHotspotAndroid);
@@ -163,6 +166,12 @@ void handleAPIv1FromRadio(HTTPRequest *req, HTTPResponse *res)
     res->setHeader("Access-Control-Allow-Methods", "GET");
     res->setHeader("X-Protobuf-Schema", "https://raw.githubusercontent.com/meshtastic/protobufs/master/meshtastic/mesh.proto");
 
+    if (req->getMethod() == "OPTIONS") {
+        res->setStatusCode(204); // Success with no content
+        // res->print(""); @todo remove
+        return;
+    }
+
     uint8_t txBuf[MAX_STREAM_BUF_SIZE];
     uint32_t len = 1;