Location for global classes?

We are utilizing some web services from multiple addons that all require the same classes to be defined. Is there a location to store class files that can be called upon from multiple addons?

Is it possible to create an addon that houses global functions and classes that can be included/called from other addons?

Lastly, if I place my classes into /core/classes would they disappear when an update is released? I'm not sure if the folder is wiped and replaced or if any non-native files would remain untouched?