usb_host: Refactor USBH and USB Host Library calls to HCD

This commit refactors the USBH and the USB Host Library in the following ways:

- USBH now presents an abstraction of an endpoint (via usbh_ep_handle_t)
    - Added separate functions to enqueue/dequeue URBs to a particular endpoint
    - USB Host Library no longer calls HCD API directly. Calls USBH endpoint API
      instead.
- Renamed "notif_cb" to "proc_req_cb" (Processing Request Callback)
    - This is to avoid confusion with FreerTOS task notifications and Host
      Library client event notifications.
    - The processing functions of each layer (i.e., "xxx_process()") request
      calls via the "proc_req_cb"
    - The main handling function (i.e., usb_host_lib_handle_events()) is
      responsible for calling the required "xxx_process()" of each layer

components/usb/private_include/hcd.h

@@ -448,13 +448,24 @@ esp_err_t hcd_pipe_set_persist_reset(hcd_pipe_handle_t pipe_hdl);
 void *hcd_pipe_get_context(hcd_pipe_handle_t pipe_hdl);
 
 /**
- * @brief Get the current sate of the pipe
+ * @brief Get the current state of the pipe
  *
  * @param pipe_hdl Pipe handle
  * @return hcd_pipe_state_t Current state of the pipe
  */
 hcd_pipe_state_t hcd_pipe_get_state(hcd_pipe_handle_t pipe_hdl);
 
+/**
+ * @brief Get the number of in-flight URBs in the pipe
+ *
+ * Returns the current number of URBs that have been enqueued (via hcd_urb_enqueue()) and have yet to be dequeued (via
+ * hcd_urb_dequeue()).
+ *
+ * @param pipe_hdl Pipe handle
+ * @return Number of in-flight URBs
+ */
+unsigned int hcd_pipe_get_num_urbs(hcd_pipe_handle_t pipe_hdl);
+
 /**
  * @brief Execute a command on a particular pipe
  *