wifi_prov_mgr_is_provisioned() function

Checks if device is provisioned This checks if Wi-Fi credentials are present on the NVS The Wi-Fi credentials are assumed to be kept in the same NVS namespace as used by esp_wifi component If one were to call esp_wifi_set_config() directly instead of going through the provisioning process, this function will still yield true (i.e. device will be found to be provisioned)