OPENTHREAD_CONFIG_DEFAULT_SED_BUFFER_SIZE macro

@def OPENTHREAD_CONFIG_DEFAULT_SED_BUFFER_SIZE Specifies the value used in emitted Connectivity TLV "Rx-off Child Buffer Size" field which indicates the guaranteed buffer capacity for all IPv6 datagrams destined to a given rx-off-when-idle child. Changing this config does not automatically adjust message buffers. Vendors should ensure their device can support the specified value based on the message buffer model used: - OT internal message pool (refer to `OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS` and `MESSAGE_BUFFER_SIZE`), or - Heap allocated message buffers (refer to `OPENTHREAD_CONFIG_MESSAGE_USE_HEAP_ENABLE), - Platform-specific message management (refer to`OPENTHREAD_CONFIG_PLATFORM_MESSAGE_MANAGEMENT`).