Add support for new File References API

The Crowdin API has been extended with new endpoints, allowing you to manage asset file references in your [Source Files](https://support.crowdin.com/developer/api/v2/#tag/Source-Files) workflow.

The Crowdin API client libraries need to be updated to include these new endpoints.

**References:**
- [List File References](https://support.crowdin.com/developer/api/v2/#tag/Source-Files/operation/api.projects.files.references.getMany)
- [Get File Reference](https://support.crowdin.com/developer/api/v2/#tag/Source-Files/operation/api.projects.files.references.get)
- [Add File Reference](https://support.crowdin.com/developer/api/v2/#tag/Source-Files/operation/api.projects.files.references.post)
- [Delete File Reference](https://support.crowdin.com/developer/api/v2/#tag/Source-Files/operation/api.projects.files.references.delete)

Start by reading the existing Go client implementations for related Source Files endpoints, then compare their patterns with the four linked Crowdin API operations. Done means the client exposes list, get, add, and delete File Reference operations with matching request and response handling; verify them against the linked API documentation and existing client tests, if present.

Written by the indexing model from the issue text.