Add support for configuration-driven IChatClient registrations

[kzu]

The following example (in this case using Tomlyn.Extensions.Configuration for .toml-formatted config) showcases configuration-driven clients for openai and xai:

[ai.clients.openai]
modelid = "gpt-4.1"
organizationid = "kzu"
projectid = "ai"
apikey = "sk-...."

[ai.clients.grok]
endpoint = "https://api.x.ai/v1"
modelid = "grok-4-fast-non-reasoning"
apikey = "xai-...."

Usage in an Azure Functions app:

var builder = FunctionsApplication.CreateBuilder(args);
builder.Configuration.AddTomlFile("ai.toml", optional: false, reloadOnChange: true);
builder.Services.UseChatClients(builder.Configuration);

var app = builder.Build();

var openai = services.GetRequiredKeyedService<IChatClient>("openai");
var xai = services.GetRequiredKeyedService<IChatClient>("grok");

The registered clients are fully reloadable when configuration changes, including swapping providers, changing default model, api key and so on.

This makes local development and testing of various settings super productive.

[kzu]

🧪 Details on Ubuntu 24.04.3 LTS

from dotnet-retest v1.0.0 on .NET 9.0.9 with 💜 by @devlooped