feat: Add File References API support

[7uyash]

Fixes #240
This commit implements support for the new Crowdin API file reference endpoints, allowing users to manage asset file references in their Source Files workflow.

Added:

• FileReference model with proper getters/setters
• listReferences(), getReference(), addReference(), deleteReference() methods in FileApi
• Comprehensive unit tests for all new functionality
• Usage examples in EXAMPLES.md

API Endpoints Supported:

• GET /projects/{projectId}/files/{fileId}/references
• GET /projects/{projectId}/files/{fileId}/references/{referenceId}
• POST /projects/{projectId}/files/{fileId}/references
• DELETE /projects/{projectId}/files/{fileId}/references/{referenceId}

Features:

• Complete CRUD operations for file references
• Enterprise API compatibility
• Full test coverage with proper mocking
• Follows existing codebase patterns and conventions
• No breaking changes - purely additive feature

Usage:

$references = $crowdin->file->listReferences($projectId, $fileId);
$reference = $crowdin->file->addReference($projectId, $fileId, [
    'name' => 'Asset Reference',
    'type' => 'image', 
    'url' => 'https://your-cdn.com/assets/image.png'
]);

[andrii-bodnar]

File references are not something so common, there is no need in the dedicated example

[Copilot]

Pull Request Overview

This PR adds comprehensive support for managing file references in the Crowdin API client, enabling users to list, get, add, and delete file references for project files.

• Introduces the FileReference model class with all necessary getters and setters
• Adds four API methods to FileApi for complete file reference CRUD operations
• Includes comprehensive test coverage for both the model and API methods
• Updates documentation with practical examples for using the new functionality

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/CrowdinApiClient/Model/FileReference.php	New model class representing a file reference with properties like id, projectId, fileId, name, type, url, and timestamps
src/CrowdinApiClient/Api/FileApi.php	Adds four new methods: listReferences, getReference, addReference, and deleteReference for managing file references
tests/CrowdinApiClient/Model/FileReferenceTest.php	Comprehensive unit tests for the FileReference model covering constructor initialization and setter methods
tests/CrowdinApiClient/Api/FileApiTest.php	Integration tests for all four file reference API methods with proper mock responses
EXAMPLES.md	Adds a complete example section demonstrating how to use the new file reference management features

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