feat(openthread) Add custom transport to OpenThread

- add a custom transport with Tx() and Rx() callbacks in OpenThread config
  - OpenThread calls Tx() to transmit, Rx() to receive data
  - external driver provides the transport for sending data between OT RCP and Host
- add an OpenThread call used by external driver to send a signal to OpenThread when there is incoming data

components/openthread/include/esp_openthread_transport.h

@@ -0,0 +1,33 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Wakes the OpenThread main loop on RCP when custom transport RX data is available.
+ *
+ * Call this from another task or ISR (after RX data has been queued for transport_rx) when using
+ * @ref esp_openthread_transport_config_t so select() returns without waiting for the full timeout.
+ *
+ */
+void esp_openthread_transport_notify_rcp_rx(void);
+
+/**
+ * @brief Wakes the OpenThread main loop on Host when custom transport RX data is available.
+ *
+ * Call this from another task or ISR (after RX data has been queued for transport_rx) when using
+ * @ref esp_openthread_transport_config_t so select() returns without waiting for the full timeout.
+ *
+ */
+void esp_openthread_transport_notify_host_rx(void);
+
+#ifdef __cplusplus
+} // end of extern "C"
+#endif