Add event hooks when NTP update is started and complete.

[stash-s]

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Hello,

I thought it would be useful and consistent with other Arduino libraries to allow triggering some actions when actual NTP conversation starts and ends.

In my case this API greatly simplifies showing NTP conversation status on my device. NTPClient::update() method is called each time in loop(), but only once per specified period actual NTP conversation is started under the hood.

I did not find any other method to figure out if update() will actually start NTP dialog before I call it in loop() block.

Best regards, Staszek

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