Add `setPoolServerIP` and `setPoolServerAddress` methods

[keriszafir]

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add setPoolServerIP (accepts IP address), setPoolServerAddress (domain as well as IP address) methods

Fixes https://github.com/arduino-libraries/NTPClient/issues/103

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[matthewdwatkins]

It appears that this PR has been held up for 1.5 years because the contributor hasn't signed the CLA. I need this functionality for a current project. Would it be possible for me to resubmit this and get it merged?

[keriszafir]

Of course :)

[matthewdwatkins]

Looks like it's already been resubmitted multiple times it just hasn't been approved yet. Too bad.