How could I create a button to loop through entries and re-save them? I've been working on a plugin that updates entry data based on the results from a json array using entries.onBeforeSaveEntry.

Since this API might put out different information in the future, I would like to create an "update all" button that would cycle through all entries and just save them rather than going through each one and clicking save manually. 

Any ideas?
If all you want to do is re-save all of the entries without making any modifications to them, you're best bet is to use the completely undocumented TaskService and trigger the ResaveElements task.

There is an example of how to do it from your plugin here.