Introduce L2CAP infrastructure.

L2CAP is the underlying technology powering GATT.
BLE 5 exposes L2CAP COC (Connection Oriented Channels)
allowing a streaming API that leads to much higher throughputs
than you can achieve with updating GATT characteristics.

The patch follows the established infrastructure very closely.
The main components are:

- `NimBLEL2CAPChannel`, encapsulating an L2CAP COC.
- `NimBLEL2CAPServer`, encapsulating the L2CAP service.
- `Examples/L2CAP`, containing a client and a server application.

Apart from these, only minor adjustments to the existing code was
necessary.

examples/L2CAP/L2CAP_Server/CMakeLists.txt

@@ -0,0 +1,7 @@
+# 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)
+set(SUPPORTED_TARGETS esp32 esp32s3 esp32c3 esp32c6)
+project(L2CAP_server)

examples/L2CAP/L2CAP_Server/Makefile

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+PROJECT_NAME := L2CAP_server
+
+include $(IDF_PATH)/make/project.mk

examples/L2CAP/L2CAP_Server/main/CMakeLists.txt

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

examples/L2CAP/L2CAP_Server/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/L2CAP/L2CAP_Server/main/idf_component.yml

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+dependencies:
+  local/esp-nimble-cpp:
+    path: ../../../../../esp-nimble-cpp/

examples/L2CAP/L2CAP_Server/main/main.cpp

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+#include <NimBLEDevice.h>
+
+// See the following for generating UUIDs:
+// https://www.uuidgenerator.net/
+
+#define SERVICE_UUID        "dcbc7255-1e9e-49a0-a360-b0430b6c6905"
+#define CHARACTERISTIC_UUID "371a55c8-f251-4ad2-90b3-c7c195b049be"
+#define L2CAP_CHANNEL        150
+#define L2CAP_MTU            5000
+
+class GATTCallbacks: public BLEServerCallbacks {
+
+public:
+    void onConnect(BLEServer* pServer, BLEConnInfo& info) {
+        /// Booster #1
+        pServer->setDataLen(info.getConnHandle(), 251);
+        /// Booster #2 (especially for Apple devices)
+        BLEDevice::getServer()->updateConnParams(info.getConnHandle(), 12, 12, 0, 200);
+    }
+};
+
+class L2CAPChannelCallbacks: public BLEL2CAPChannelCallbacks {
+
+public:
+    bool connected = false;
+    size_t numberOfReceivedBytes;
+    uint8_t nextSequenceNumber;
+
+public:
+    void onConnect(NimBLEL2CAPChannel* channel) {
+        printf("L2CAP connection established\n");
+        connected = true;
+        numberOfReceivedBytes = nextSequenceNumber = 0;
+    }
+
+    void onRead(NimBLEL2CAPChannel* channel, std::vector<uint8_t>& data) {
+        numberOfReceivedBytes += data.size();
+        size_t sequenceNumber = data[0];
+        printf("L2CAP read %d bytes w/ sequence number %d", data.size(), sequenceNumber);
+        if (sequenceNumber != nextSequenceNumber) {
+            printf("(wrong sequence number %d, expected %d)\n", sequenceNumber, nextSequenceNumber);
+        } else {
+            printf("\n");
+            nextSequenceNumber++;
+        }
+    }
+    void onDisconnect(NimBLEL2CAPChannel* channel) {
+        printf("L2CAP disconnected\n");
+        connected = false;
+    }
+};
+
+extern "C"
+void app_main(void) {
+    printf("Starting L2CAP server example [%lu free] [%lu min]\n", esp_get_free_heap_size(), esp_get_minimum_free_heap_size());
+
+    BLEDevice::init("L2CAP-Server");
+    BLEDevice::setMTU(BLE_ATT_MTU_MAX);
+
+    auto cocServer = BLEDevice::createL2CAPServer();
+    auto l2capChannelCallbacks = new L2CAPChannelCallbacks();
+    auto channel = cocServer->createService(L2CAP_CHANNEL, L2CAP_MTU, l2capChannelCallbacks);
+    
+    auto server = BLEDevice::createServer();
+    server->setCallbacks(new GATTCallbacks());
+    auto service = server->createService(SERVICE_UUID);
+    auto characteristic = service->createCharacteristic(CHARACTERISTIC_UUID, NIMBLE_PROPERTY::READ);
+    characteristic->setValue(L2CAP_CHANNEL);
+    service->start();
+    auto advertising = BLEDevice::getAdvertising();
+    advertising->addServiceUUID(SERVICE_UUID);
+    advertising->enableScanResponse(true);
+
+    BLEDevice::startAdvertising();
+    printf("Server waiting for connection requests [%lu free] [%lu min]\n", esp_get_free_heap_size(), esp_get_minimum_free_heap_size());
+
+    // Wait until transfer actually starts...
+    while (!l2capChannelCallbacks->numberOfReceivedBytes) {
+        vTaskDelay(10 / portTICK_PERIOD_MS);
+    }
+    printf("\n\n\n");
+    int numberOfSeconds = 0;
+
+    while (true) {
+        vTaskDelay(1000 / portTICK_PERIOD_MS);
+        if (!l2capChannelCallbacks->connected) { continue; }
+        int bps = l2capChannelCallbacks->numberOfReceivedBytes / ++numberOfSeconds;
+        printf("Bandwidth: %d b/sec = %d KB/sec [%lu free] [%lu min]\n", bps, bps / 1024, esp_get_free_heap_size(), esp_get_minimum_free_heap_size());
+    }
+}

examples/L2CAP/L2CAP_Server/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
+CONFIG_BT_NIMBLE_L2CAP_COC_MAX_NUM=1