Add comprehensive dependency injection support with IServiceCollection extension methods

[mokarchi]

This PR implements enhanced dependency injection support for the OpenAI .NET library, addressing the feature request for built-in extension methods for IServiceCollection. The implementation follows .NET ecosystem standards similar to Azure SDKs, making it easier to integrate OpenAI clients into ASP.NET Core, Blazor, and other .NET applications.

Key Features Added

Extension Methods for IServiceCollection

The library now provides convenient extension methods for registering OpenAI clients:

using OpenAI.Extensions.DependencyInjection;

// Register individual clients with API key
builder.Services.AddOpenAIChat("gpt-4o", "your-api-key");
builder.Services.AddOpenAIEmbeddings("text-embedding-3-small");

// Register the main OpenAI client factory
builder.Services.AddOpenAI("your-api-key");
builder.Services.AddOpenAIChat("gpt-4o"); // Uses the registered OpenAIClient

Configuration Integration

Full support for IConfiguration binding with appsettings.json:

{
  "OpenAI": {
    "ApiKey": "your-api-key",
    "DefaultChatModel": "gpt-4o",
    "DefaultEmbeddingModel": "text-embedding-3-small",
    "Endpoint": "https://api.openai.com/v1",
    "OrganizationId": "your-org-id"
  }
}

// Register services from configuration
builder.Services.AddOpenAIFromConfiguration(builder.Configuration);

// Add specific clients using default models from configuration
builder.Services.AddChatClientFromConfiguration();
builder.Services.AddEmbeddingClientFromConfiguration();

// Or add all common clients at once
builder.Services.AddAllOpenAIClientsFromConfiguration();

Environment Variable Support

Automatic fallback to the OPENAI_API_KEY environment variable for development scenarios:

// This will use the OPENAI_API_KEY environment variable if no API key is provided
builder.Services.AddOpenAIChat("gpt-4o");

Implementation Details

Available Extension Methods

• Core Registration: AddOpenAI(), AddOpenAIChat(), AddOpenAIEmbeddings(), AddOpenAIAudio(), AddOpenAIImages(), AddOpenAIModeration()
• Configuration-Based: AddOpenAIFromConfiguration(), AddChatClientFromConfiguration(), etc.
• Bulk Registration: AddAllOpenAIClientsFromConfiguration()

Enhanced Configuration Options

The new OpenAIServiceOptions class extends OpenAIClientOptions with DI-specific settings:

• Default model configurations for each client type
• API key configuration with environment variable fallback
• All existing OpenAIClientOptions properties (Endpoint, OrganizationId, ProjectId, etc.)

Thread-Safe Singleton Registration

All clients are registered as singletons by default, following the existing recommendation in the README. This maximizes resource efficiency and HTTP connection reuse while maintaining thread safety.

Backward Compatibility

This implementation is fully backward compatible. Existing manual registration patterns continue to work unchanged:

// Existing manual registration still works
builder.Services.AddSingleton<ChatClient>(serviceProvider =>
{
    var apiKey = Environment.GetEnvironmentVariable("OPENAI_API_KEY");
    return new ChatClient("gpt-4o", apiKey);
});

[stephentoub]

Should this instead be left to Aspire, e.g. https://www.nuget.org/packages/Aspire.OpenAI ?

cc: @eerhardt

[KrzysztofCwalina]

I also think configuration like this needs to be aligned with what we already have in both Aspire, .NET, and Azure. In fact, I had a sync up with @eerhardt a couple of days ago to design a consistent solution.

cc @m-nash