Added Go implementation of Files API with test suite

[cestercian]

Overview

This PR adds a Go implementation of the Files API, following the structure of the Python and JavaScript implementations. It includes both functionality (files.go) and test cases (files_test.go).

Changes

• Implemented files.go with:
  • Text, image, audio, and video file upload & processing
  • PDF file upload & processing
  • File listing, retrieval, and deletion
• Created files_test.go with test cases for each function
• Ensured consistent structure, comments, and licensing across implementations
• Added proper error handling and helper functions for media access

Testing

• Tests are designed to be run with the Go test framework
• Automatically skipped if the API key is not set

Documentation

• Functions include structured comments and START/END markers for consistency with Python and JavaScript implementations