Add asynchronous client connection secure

* Adds parameter `rcPtr` to `NimBLEDevice::startSecurity`, default value works as the original method.
* * `rcPtr`: if not nullptr, will allow caller to obtain the internal return code.
* Adds parameter `async` to `NimBLEClient::secureConnection`, default value works as the original method.
* * `async`; if true, will send the secure command and return immediately with a true value for successfully sending the command, else false.

src/NimBLEDevice.cpp

@@ -1136,14 +1136,17 @@ uint32_t NimBLEDevice::getSecurityPasskey() {
 /**
  * @brief Start the connection securing and authorization for this connection.
  * @param connHandle The connection handle of the peer device.
- * @returns NimBLE stack return code, 0 = success.
+ * @param rcPtr Optional pointer to pass the return code to the caller.
+ * @returns True if successfully started, success = 0 or BLE_HS_EALREADY.
  */
-bool NimBLEDevice::startSecurity(uint16_t connHandle) {
+bool NimBLEDevice::startSecurity(uint16_t connHandle, int* rcPtr) {
     int rc = ble_gap_security_initiate(connHandle);
     if (rc != 0) {
         NIMBLE_LOGE(LOG_TAG, "ble_gap_security_initiate: rc=%d %s", rc, NimBLEUtils::returnCodeToString(rc));
     }
-
+    if (rcPtr) {
+        *rcPtr = rc;
+    }
     return rc == 0 || rc == BLE_HS_EALREADY;
 } // startSecurity