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.

src/NimBLEDevice.cpp

@@ -405,7 +405,7 @@ NimBLEClient* NimBLEDevice::getClientByPeerAddress(const NimBLEAddress& addr) {
 } // getClientPeerAddress
 
 /**
- * @brief Finds the first disconnected client in the list.
+ * @brief Finds the first disconnected client available.
  * @return A pointer to the first client object that is not connected to a peer or nullptr.
  */
 NimBLEClient* NimBLEDevice::getDisconnectedClient() {
@@ -418,6 +418,21 @@ NimBLEClient* NimBLEDevice::getDisconnectedClient() {
     return nullptr;
 } // getDisconnectedClient
 
+/**
+ * @brief Get a list of connected clients.
+ * @return A vector of connected client objects.
+ */
+std::vector<NimBLEClient*> NimBLEDevice::getConnectedClients() {
+    std::vector<NimBLEClient*> clients;
+    for (const auto clt : m_pClients) {
+        if (clt != nullptr && clt->isConnected()) {
+            clients.push_back(clt);
+        }
+    }
+
+    return clients;
+} // getConnectedClients
+
 # endif // #if defined(CONFIG_BT_NIMBLE_ROLE_CENTRAL)
 
 /* -------------------------------------------------------------------------- */