Add OTA type to onStart callback

[andig]

Allows to decide if unmounting SPIFFS and potentially writing config files to SPIFFS is necessary.

Mentioned this on gitter- right now none of the examples show use of SPIFFS.end() nor is it possible to decide if thats even needed. Idea of this PR is to tell the handler what type of spiffs update is actually doing on.

If updating the handler is undesired I could also a getOTAType function or similar that would expose the _cmd value.

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[me-no-dev]

maybe instead of changing the current callback API, you can just add another method to ArduinoOTA like so:

int getCommand(){ return _cmd; }

and then check for that in the regular callback

[andig]

maybe instead of changing the current callback API, you can just add another method to ArduinoOTA like so:

Had this as an after thought, too and like it better. I'm unsure however if we need any precaution for when it's called- seems to be meaningful only after onStart has been called. Just add this to the docblock?

[me-no-dev]

sounds good to me :)