Time fix for packet, and filter addition

[henriberisha]

Stefan, thank you for this library.

Below are two code changes that i made to improve the results of all examples that you provide.

1. Fix time issue when writing or sending a packet. Change the following lines:
   uint32_t timestamp = now(); //current timestamp
uint32_t microseconds = (unsigned int)(micros() - millis() * 1000); //micro seconds offset (0 - 999)

into

uint32_t timestamp = now(); // current timestamp
uint32_t microseconds = (unsigned int)(micros() % 1000000U);

This will fix the relative time between packets, see below results:

2. Can add filter to capture management and data packets:

PCAP pcap = PCAP();
int ch = CHANNEL;
unsigned long lastChannelChange = 0;

const wifi_promiscuous_filter_t filter_data_mgmt = {
    .filter_mask = WIFI_PROMIS_FILTER_MASK_DATA | WIFI_PROMIS_FILTER_MASK_MGMT}; // this filter either DATA or MGMT frames```

```/* setup wifi */
nvs_flash_init();  
tcpip_adapter_init();  
ESP_ERROR_CHECK(esp_event_loop_init(event_handler, NULL));  
wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();  
ESP_ERROR_CHECK(esp_wifi_init(&cfg));  
ESP_ERROR_CHECK(esp_wifi_set_storage(WIFI_STORAGE_RAM)); ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_NULL));  
ESP_ERROR_CHECK(esp_wifi_start());  
esp_wifi_set_promiscuous_filter(&filter_data_mgmt); // the filter is used here  
esp_wifi_set_promiscuous(true);  
esp_wifi_set_promiscuous_rx_cb(sniffer);  
wifi_second_chan_t secondCh = (wifi_second_chan_t)NULL;  
esp_wifi_set_channel(ch, secondCh);```