add tud_speed_get()

- define both fs and hs configuration descriptor
- rename CFG_TUD_CDC_EPSIZE to CFG_TUD_CDC_EP_BUFSIZE with default size of 64 for FS, and 512 for HS

src/device/usbd.h

@@ -53,6 +53,9 @@ bool tud_task_event_ready(void);
 // Interrupt handler, name alias to DCD
 #define tud_int_handler   dcd_int_handler
 
+// Get current bus speed
+tusb_speed_t tud_speed_get(void);
+
 // Check if device is connected and configured
 bool tud_mounted(void);