.Net: Added support for OpenAI image detail level property

[dmytrostruk]

Motivation and Context

Resolves: #4759

This PR adds a support for the detail property in the OpenAI image API. The property can be configured using ImageContent.Metadata property.

ImageContent.Metadata usage:

chatHistory.AddUserMessage(
[
    new TextContent("What’s in this image?"),
    new ImageContent(imageBytes, "image/jpg") { Metadata = new Dictionary<string, object?> { ["ChatImageDetailLevel"] = "high" } }
]);

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄