Add fine-tuning

[Krivoblotsky]

Extend the library by adding fine-tuning capabilities.
Add files and fine-tunes API calls so the library supports model tuning and file retrieval.

Documentation

• Files
• Fine-Tunes

[DJBen]

Given there are multiple feature additions, I suggest assigning a priority. For example, fine-tuning would be less of a common use case on Swift (unless convinced otherwise).