The UpdateClass in the Updater component has the ability to update data to
a SPIFFS partition. It selects the first available partition using the
ESP-IDF esp_partition_find_first() function.
That behaviour is problematic if one has multiple SPIFFS partitions.
This change allows a user to pass the label argument (defaults to NULL)
to UpdateClass::begin() so a specific SPIFFS partition can be updated.
Additionally, ArduinoOTA can set this partition label using the
new method ArduinoOTAClass::setPartitionLabel which is optional.
This change does not break compatibility.
The wifi stack initialisation must be complete before calling `_udp_ota.parsePacket()` otherwise you just get a screen filled with
```
```
and due to the more async methods in ESP32 `handle()` may be called before this can occur.
* ArduinoOTA would stop receiving any packets if the port received a zero-length UDP packet, commonly sent by network scanners like nmap. Fixed to flush() after every call to parsePacket(), even if read length is 0.
Additionally, added length checking to fix a potential buffer overflow in parseInt().
Finally, added an end() method that stops the OTA listener and releases resources.
* Only end MDNS in end() if mdns mode is enabled.
