Enable Microsoft Dependency Injection container support with HTTP client factory integration  #2319

Description

• RestSharp needs to integrate with Microsoft's Dependency Injection container to support modern .NET applications using the standard DI patterns
• HTTP client lifecycle management should leverage the built-in IHttpClientFactory for proper resource handling and connection pooling
• Test infrastructure should be refactored to support multiple client instantiation patterns while maintaining consistent test coverage

Deliverables

• DI Extension Library: Create RestSharp.Extensions.DependencyInjection package with ServiceCollectionExtensions providing AddRestClient() methods for various configuration scenarios
• HTTP Client Factory: Integrate IHttpClientFactory for managed HttpClient creation and configuration through the DI container
• Shared Test Base: Extract common request test cases into RequestTestsBase to enable test reuse across different client instantiation patterns
• Test Coverage: Implement DI-specific test suite validating RestClient registration and execution through the dependency injection container