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_Client/sdkconfig.defaults

@@ -0,0 +1,13 @@
+# 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