[Question] How to add file to PBXNativeTarget

I'm trying to add a file (GoogleServices-Info.plist) to my main project folder, but can't figure out what function to use. I've tried every one, and the best I can do is get it into my Plugins folder. Anyone know?

I thought these might work, but they didn't:
```javascript
project.addToPbxGroupType(file, targetKey, 'PBXNativeTarget');
project.addToPbxFileReferenceSection(file);

// this one gives an error; sources is undefined
project.addToPbxCopyfilesBuildPhase(file);
```

Start by reviewing the project's APIs for PBX groups, file references, and copy-files build phases, then reproduce the reported behavior with GoogleServices-Info.plist and a PBXNativeTarget. The issue is complete when the correct target and project folder contain the file without the reported error, with a focused test or documented usage demonstrating it.

Written by the indexing model from the issue text.