1. provide options for bluetooth low power mode
2. provide two options for bluetooth low power clock: main XTAL and external 32kHz XTAL
3. provide function and callbacks to control bluetooth low power mode, including enable/disable sleep, software wakeup request, low power clock settings, check power state, etc
4. modify vhci API vhci_host_send_packet to use blocking mode
5. note that DFS and bluetooth modem sleep can not be used together currently.
1. fix the bug. Modify the condition that esp_bt_controller_mem_release() shoud be only called before esp_bt_controller_init() or after esp_bt_controller_deinit()
2. modify the example to use esp_bt_controller_mem_release()
