Add asyncronous client connect and MTU exchange.

* Adds parameters `asyncConnect` and `exchangeMTU` to `NimBLEClient::connect`, default values work as the original connect method.
* * `asyncConnect`; if true, will send the connect command and return immediately with a true value for successfully sending the command, else false.
* * `exchangeMTU`; if true will send the exchange MTU command upon connection, otherwise not and the application can choose to do this later via the `exchangeMTU` method.
* Adds `onMTUChange` callback to `NimBLEClientCallbacks`
* Add `NimBLEDevice::getConnectedClients()` which returns a vector of pointers to the currently connected client instances.
* Calling `NimBLEClient::connect` will no longer cancel already in progress connections.

examples/NimBLE_Async_Client/CMakeLists.txt

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+# The following lines of boilerplate have to be in your project's
+# CMakeLists in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.5)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(NimBLE_Async_Client)

examples/NimBLE_Async_Client/Makefile

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+PROJECT_NAME := NimBLE_Async_Client
+
+include $(IDF_PATH)/make/project.mk

examples/NimBLE_Async_Client/main/CMakeLists.txt

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+set(COMPONENT_SRCS "main.cpp")
+set(COMPONENT_ADD_INCLUDEDIRS ".")
+
+register_component()

examples/NimBLE_Async_Client/main/component.mk

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+#
+# "main" pseudo-component makefile.
+#
+# (Uses default behaviour of compiling all source files in directory, adding 'include' to include path.)

examples/NimBLE_Async_Client/main/main.cpp

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+
+/**
+ *  NimBLE_Async_client Demo:
+ *
+ *  Demonstrates asynchronous client operations.
+ *
+ *  Created: on November 4, 2024
+ *      Author: H2zero
+ *
+ */
+
+#include <NimBLEDevice.h>
+
+static constexpr uint32_t scanTimeMs = 5 * 1000;
+
+class ClientCallbacks : public NimBLEClientCallbacks {
+    void onConnect(NimBLEClient* pClient) {
+        printf("Connected to: %s\n", pClient->getPeerAddress().toString().c_str());
+    }
+
+    void onDisconnect(NimBLEClient* pClient, int reason) {
+        printf("%s Disconnected, reason = %d - Starting scan\n", pClient->getPeerAddress().toString().c_str(), reason);
+        NimBLEDevice::getScan()->start(scanTimeMs);
+    }
+} clientCB;
+
+class scanCallbacks : public NimBLEScanCallbacks {
+    void onResult(NimBLEAdvertisedDevice* advertisedDevice) {
+        printf("Advertised Device found: %s\n", advertisedDevice->toString().c_str());
+        if (advertisedDevice->haveName() && advertisedDevice->getName() == "NimBLE-Server") {
+            printf("Found Our Device\n");
+
+            auto pClient = NimBLEDevice::getDisconnectedClient();
+            if (!pClient) {
+                pClient = NimBLEDevice::createClient(advertisedDevice->getAddress());
+                if (!pClient) {
+                    printf("Failed to create client\n");
+                    return;
+                }
+            }
+
+            pClient->setClientCallbacks(&clientCB, false);
+            if (!pClient->connect(true, true, false)) { // delete attributes, async connect, no MTU exchange
+                NimBLEDevice::deleteClient(pClient);
+                printf("Failed to connect\n");
+                return;
+            }
+        }
+    }
+
+    void onScanEnd(NimBLEScanResults results) {
+        printf("Scan Ended\n");
+        NimBLEDevice::getScan()->start(scanTimeMs);
+    }
+};
+
+extern "C" void app_main(void) {
+    printf("Starting NimBLE Async Client\n");
+    NimBLEDevice::init("");
+    NimBLEDevice::setPower(3); /** +3db */
+
+    NimBLEScan* pScan = NimBLEDevice::getScan();
+    pScan->setScanCallbacks(new scanCallbacks());
+    pScan->setInterval(45);
+    pScan->setWindow(15);
+    pScan->setActiveScan(true);
+    pScan->start(scanTimeMs);
+
+    for (;;) {
+        vTaskDelay(pdMS_TO_TICKS(1000));
+        auto pClients = NimBLEDevice::getConnectedClients();
+        if (!pClients.size()) {
+            continue;
+        }
+
+        for (auto& pClient : pClients) {
+            printf("%s\n", pClient->toString().c_str());
+            NimBLEDevice::deleteClient(pClient);
+        }
+
+        NimBLEDevice::getScan()->start(scanTimeMs);
+    }
+}

examples/NimBLE_Async_Client/sdkconfig.defaults

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+# Override some defaults so BT stack is enabled
+# in this example
+
+#
+# BT config
+#
+CONFIG_BT_ENABLED=y
+CONFIG_BTDM_CTRL_MODE_BLE_ONLY=y
+CONFIG_BTDM_CTRL_MODE_BR_EDR_ONLY=n
+CONFIG_BTDM_CTRL_MODE_BTDM=n
+CONFIG_BT_BLUEDROID_ENABLED=n
+CONFIG_BT_NIMBLE_ENABLED=y