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.
The current OpenThread initialization is too verbose and error-prone.
Using a simple flow will be more user friendly and paves way for
introducing more border router features.
