fix: path of result image was missing file:// prefix(ios) (#59)

## Description
Result path returned from style transfer was missing file:// prefix, due
to that using style transfered image for other task was throwing error

### Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Documentation update (improves or adds clarity to existing
documentation)

### Tested on
- [x] iOS
- [ ] Android

### Testing instructions
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Include setup details if necessary. -->

### Screenshots
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### Related issues
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### Checklist
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have updated the documentation accordingly
- [ ] My changes generate no new warnings

### Additional notes
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reviewers might need to understand this PR. -->

Author: NorbertKlockiewicz

ios/RnExecutorch/utils/ImageProcessor.mm

@@ -54,7 +54,7 @@ + (NSString *)saveToTempFile:(const cv::Mat&)image {
                                  userInfo:nil];
   }
 
-  return outputPath;
+  return [NSString stringWithFormat:@"file://%@", outputPath];
 }
 
 + (cv::Mat)readImage:(NSString *)source {