Enhancement request: Simpler multiple DDNS support

[anishsane]

I might have a need to enable multiple DDNS services together.

Currently, I can achieve it by multiple objects of EasyDDNSClass and initializing all of them separately.
However, a lot of the code is common:

1. Update time expiry
2. Getting the public IP
   etc

It will be useful if we could keep the common members (e.g. new_ip; old_ip;) and methods as static to class and update only once, even if there are many services defined.

Also, from a usability point of view, It will be very useful if (in addition to the current single service support), we can init the library via an array or something.

Something like:

String myServices[] = ["noip", "duckdns", "duckdns"];
EasyDDNS.service(myServices);

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