[th/nm-device-utils] core: add "nm-device-utils.[ch]" with simple helper functions

Thomas Haller requested to merge th/nm-device-utils into main

"nm-device.c" is huge, and it does complicated things like handling the state of the device and IP configuration.

It also contains simpler, individual functions, like converting enums to strings. Let's move those trivial functions to a new module, so that the remaining part is smaller.

"nm-device-utils.[ch]" should only contain simpler functions that have no complex behavior or state.

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