Add IHttpClientFactory integration for ASP.NET Core compatibility

[tw4]

Summary

Nexar creates its own HttpClient instances internally, which is not compatible with the recommended IHttpClientFactory pattern in ASP.NET Core. This leads to socket exhaustion issues in long-running applications.

Problem

• No DI (Dependency Injection) friendly constructor
• Cannot plug into named or typed HTTP clients registered via IHttpClientFactory
• Not compatible with ASP.NET Core's built-in resilience and retry pipelines (e.g., Polly)

Proposed Solution

• Add a constructor overload that accepts an HttpClient instance
• Provide extension methods for IServiceCollection to register Nexar with DI

// Registration
services.AddNexar(config => {
    config.BaseUrl = "https://api.example.com";
});

// Usage via DI
public class MyService(INexar nexar) { }