[2.0.0] BtClassic Discovery with info without connect (#4811)

Hey guys,
so I wanted to do a BtClassic Discovery without the need to call connect
and to list all found devices on a display and continue work with that list.

I wasn't capable to test the example code with my file structure, but I did use the discovery already in some different situations.

However when I noted that the Bluedroid stack won't let me enforce an RfComm SPP connection to a GPS Device (Skytraxx 2 plus, I guess its interface is built so simple that it doesn't advertise its SPP over SDP), I will probably have to switch to BtStack (BlueKitchen) and stop on this side meanwhile

libraries/BluetoothSerial/examples/bt_classic_device_discovery/bt_classic_device_discovery.ino

@@ -0,0 +1,52 @@
+#include <BluetoothSerial.h>
+
+#if !defined(CONFIG_BT_ENABLED) || !defined(CONFIG_BLUEDROID_ENABLED)
+#error Bluetooth is not enabled! Please run `make menuconfig` to and enable it
+#endif
+
+BluetoothSerial SerialBT;
+
+
+#define BT_DISCOVER_TIME	10000
+
+
+static bool btScanAsync = true;
+static bool btScanSync = true;
+
+
+void btAdvertisedDeviceFound(BTAdvertisedDevice* pDevice) {
+	Serial.printf("Found a device asynchronously: %s\n", pDevice->toString().c_str());
+}
+
+void setup() {
+  Serial.begin(115200);
+  SerialBT.begin("ESP32test"); //Bluetooth device name
+  Serial.println("The device started, now you can pair it with bluetooth!");
+
+
+  if (btScanAsync) {
+    Serial.print("Starting discoverAsync...");
+    if (SerialBT.discoverAsync(btAdvertisedDeviceFound)) {
+      Serial.println("Findings will be reported in \"btAdvertisedDeviceFound\"");
+      delay(10000);
+      Serial.print("Stopping discoverAsync... ");
+      SerialBT.discoverAsyncStop();
+      Serial.println("stopped");
+    } else {
+      Serial.println("Error on discoverAsync f.e. not workin after a \"connect\"");
+    }
+  }
+  
+  if (btScanSync) {
+    Serial.println("Starting discover...");
+    BTScanResults *pResults = SerialBT.discover(BT_DISCOVER_TIME);
+    if (pResults)
+      pResults->dump(&Serial);
+    else
+      Serial.println("Error on BT Scan, no result!");
+  }
+}
+
+void loop() {
+  delay(100);
+}