openthread: add platform UDP and border router example

This MR adds the OpenThread border router example and supports the
Thread 1.1 border agent feature.

* Adds the OPENTHREAD_BORDER_ROUTER Kconfig option.
* Adds platform UDP and task queue port for the border agent feature.
* Adds `esp_openthread_border_router_*` api.
* Adds the `esp_otbr` example.

components/openthread/private_include/esp_openthread_task_queue.h

@@ -0,0 +1,87 @@
+// Copyright 2021 Espressif Systems (Shanghai) CO LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License
+
+#pragma once
+
+#include "esp_err.h"
+#include "esp_openthread.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+* @brief OpenThread task declaration
+*
+*/
+typedef void (*esp_openthread_task_t)(void *);
+
+/**
+* @brief This function allocs and initializes the OpenThread task queue.
+*
+* @return
+*   - ESP_OK on success
+*   - ESP_ERR_NO_MEM on queue allocation failure
+*   - ESP_FAIL on other failures
+*
+*/
+esp_err_t esp_openthread_task_queue_init(void);
+
+/**
+* @brief This function posts a task to the OpenThread task queue.
+*
+* @param[in] task   The task to execute.
+* @param[in] arg    The context argument to be passed to the task.
+*
+* @return
+*   - ESP_OK
+*   - ESP_FAIL
+*
+*/
+esp_err_t esp_openthread_task_queue_post(esp_openthread_task_t task, void *arg);
+
+/**
+* @brief This function updates the task queue inner fd to the main loop.
+*
+* @param[inout]    mainloop    The main loop context.
+*
+*/
+void esp_openthread_task_queue_update(esp_openthread_mainloop_context_t *mainloop);
+
+/**
+ * @brief This function drives the execution of the task queue.
+ *
+ * @param[in]    instance   The OpenThread instance.
+ * @param[in]    mainloop   The main loop context.
+ *
+ * @return
+ *      - ESP_OK
+ *      - ESP_FAIL
+ *
+ */
+esp_err_t esp_openthread_task_queue_process(otInstance *instance, const esp_openthread_mainloop_context_t *mainloop);
+
+/**
+ * @brief This function deinitializes the task queue.
+ *
+ * @return
+ *      - ESP_OK
+ *      - ESP_FAIL
+ *
+ */
+esp_err_t esp_openthread_task_queue_deinit(void);
+
+#ifdef __cplusplus
+}
+#endif