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.

src/NimBLEL2CAPChannel.h

@@ -0,0 +1,120 @@
+//
+// (C) Dr. Michael 'Mickey' Lauer <mickey@vanille-media.de>
+//
+#pragma once
+#ifndef NIMBLEL2CAPCHANNEL_H
+#define NIMBLEL2CAPCHANNEL_H
+
+#include "inttypes.h"
+#include "host/ble_l2cap.h"
+#include "os/os_mbuf.h"
+#include "os/os_mempool.h"
+
+/****  FIX COMPILATION ****/
+# undef min
+# undef max
+/**************************/
+
+#include <vector>
+#include <atomic>
+
+class NimBLEClient;
+class NimBLEL2CAPChannelCallbacks;
+
+/**
+ * @brief Encapsulates a L2CAP channel.
+ * 
+ * This class is used to encapsulate a L2CAP connection oriented channel, both
+ * from the "server" (which waits for the connection to be opened) and the "client"
+ * (which opens the connection) point of view.
+ */
+class NimBLEL2CAPChannel {
+
+public:
+    /// @brief Open an L2CAP channel via the specified PSM and MTU.
+    /// @param[in] psm The PSM to use.
+    /// @param[in] mtu The MTU to use. Note that this is the local MTU. Upon opening the channel,
+    /// the final MTU will be negotiated to be the minimum of local and remote.
+    /// @param[in] callbacks The callbacks to use. NOTE that these callbacks are called from the
+    /// context of the NimBLE bluetooth task (`nimble_host`) and MUST be handled as fast as possible.
+    /// @return True if the channel was opened successfully, false otherwise.
+    static NimBLEL2CAPChannel* connect(NimBLEClient* client, uint16_t psm, uint16_t mtu, NimBLEL2CAPChannelCallbacks* callbacks);
+
+    /// @brief Write data to the channel.
+    ///
+    /// If the size of the data exceeds the MTU, the data will be split into multiple fragments.
+    /// @return true on success, after the data has been sent.
+    /// @return false, if the data can't be sent.
+    ///
+    /// NOTE: This function will block until the data has been sent or an error occurred.
+    bool write(const std::vector<uint8_t>& bytes);
+
+    /// @return True, if the channel is connected. False, otherwise.
+    bool isConnected() const { return !!channel; }
+
+protected:
+
+    NimBLEL2CAPChannel(uint16_t psm, uint16_t mtu, NimBLEL2CAPChannelCallbacks* callbacks);
+    ~NimBLEL2CAPChannel();
+
+    int handleConnectionEvent(struct ble_l2cap_event* event);
+    int handleAcceptEvent(struct ble_l2cap_event* event);
+    int handleDataReceivedEvent(struct ble_l2cap_event* event);
+    int handleTxUnstalledEvent(struct ble_l2cap_event* event);
+    int handleDisconnectionEvent(struct ble_l2cap_event* event);
+
+private:
+    friend class NimBLEL2CAPServer;
+    static constexpr const char* LOG_TAG = "NimBLEL2CAPChannel";
+
+    const uint16_t psm; // PSM of the channel
+    const uint16_t mtu; // The requested (local) MTU of the channel, might be larger than negotiated MTU
+    struct ble_l2cap_chan* channel = nullptr;
+    NimBLEL2CAPChannelCallbacks* callbacks;
+    uint8_t* receiveBuffer = nullptr; // buffers a full (local) MTU
+
+    // NimBLE memory pool
+    void* _coc_memory = nullptr;
+    struct os_mempool _coc_mempool;
+    struct os_mbuf_pool _coc_mbuf_pool;
+
+    // Runtime handling
+    std::atomic<bool> stalled{false};
+    SemaphoreHandle_t stalledSemaphore = nullptr;
+
+    // Allocate / deallocate NimBLE memory pool
+    bool setupMemPool();
+    void teardownMemPool();
+
+    // Writes data up to the size of the negotiated MTU to the channel.
+    int writeFragment(std::vector<uint8_t>::const_iterator begin, std::vector<uint8_t>::const_iterator end);
+
+    // L2CAP event handler
+    static int handleL2capEvent(struct ble_l2cap_event* event, void *arg);
+};
+
+/**
+ * @brief Callbacks base class for the L2CAP channel.
+ */
+class NimBLEL2CAPChannelCallbacks {
+
+public:
+    NimBLEL2CAPChannelCallbacks() = default;
+    virtual ~NimBLEL2CAPChannelCallbacks() = default;
+
+    /// Called when the client attempts to open a channel on the server.
+    /// You can choose to accept or deny the connection.
+    /// Default implementation returns true.
+    virtual bool shouldAcceptConnection(NimBLEL2CAPChannel* channel) { return true; }
+    /// Called after a connection has been made.
+    /// Default implementation does nothing.
+    virtual void onConnect(NimBLEL2CAPChannel* channel, uint16_t negotiatedMTU) {};
+    /// Called when data has been read from the channel.
+    /// Default implementation does nothing.
+    virtual void onRead(NimBLEL2CAPChannel* channel, std::vector<uint8_t>& data) {};
+    /// Called after the channel has been disconnected.
+    /// Default implementation does nothing.
+    virtual void onDisconnect(NimBLEL2CAPChannel* channel) {};
+};
+
+#endif