Feature request for file changes event

This is a feature request.

Currently it seems that we don't receive any events for changed files with their diff, which makes it difficult with the SDK to report nicely what GitHub Copilot has changed during a prompt iteration.

An example of such API in C# could be:

```csharp
public partial class AssistantFileChangesEvent : SessionEvent
{
[JsonIgnore]
public override string Type => "assistant.file_changes";

[JsonPropertyName("changes")]
public required List Changes { get; set; }
}

public partial class AssistantFileChangeData
{
[JsonPropertyName("diff")]
public string Diff { get; set; } = string.Empty;
[JsonPropertyName("kind")]
public AssistantFileChangeKind Kind { get; set; } = default!;
[JsonPropertyName("path")]
public string Path { get; set; } = string.Empty;
[JsonPropertyName("move_path")]
public string MovePath { get; set; } = string.Empty; // When kind == "update"
}

[JsonConverter(typeof(JsonStringEnumConverter))]
public enum AssistantFileChangeKind
{
[JsonStringEnumMemberName("add")]
Add,
[JsonStringEnumMemberName("delete")]
Delete,
[JsonStringEnumMemberName("update")]
Update, // Use "move_path"
}
```

It would allow to display more nicely during a user/assistant iteration the files that were modified.