Added Pre OTA Update Callback

[anindyamitra15]

If someone needs to run some code just before the update begins, it can be done by adding the line: AsyncElegantOTA.preFotaRoutineCallback(callback); to the setup. All the instructions are written under Documentation in
README
Tested this with ESP8266 as well as ESP32, works on both.

[ayushsharma82]

Hi @anindyamitra15 ,
Thanks a lot for this addition.

Can you rename it to onOTAStart ?

If possible, please replicate the same for ElegantOTA library as well. These 2 libraries go hand in hand in regards to functionality.

[anindyamitra15]

Hi @anindyamitra15 , Thanks a lot for this addition.

Can you rename it to onOTAStart ?

If possible, please replicate the same for ElegantOTA library as well. These 2 libraries go hand in hand in regards to functionality.

Ok I will rename it to onOTAStart
And then I'll also implement it on the non-async version as well

[anindyamitra15]

All requested change(s) for this repository has been made. Kindly let me know if there are more changes to be done.
I'll proceed with ElegantOTA soon.

[anindyamitra15]

@ayushsharma82