Python: [Feature]: dotnet, python: agent support genimage model

[williamlzw]

Description

[Feature]: dotnet, python: agent support genimage model

`https://github.com/dotnet/extensions/blob/main/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIImageGeneratorTests.cs```

Code Sample

var client = new OpenAIClient(new ApiKeyCredential("key"), new OpenAIClientOptions { Endpoint = endpoint });

IImageGenerator imageClient = client.GetImageClient(model).AsIImageGenerator();

Language/SDK

Both