Add DI package documentation and new RestClient configuration extension

[qodo-merge-for-open-source]

Description

• The DI package lacks user-facing documentation explaining integration with Microsoft.Extensions.DependencyInjection
• Users need multiple ways to configure RestClient instances with different option patterns
• Package README should be distributed with NuGet package for discoverability

Deliverables

• Documentation: Create comprehensive README.md with usage examples for default, base URL, and custom options registration
• Extension Method: Add new AddRestClient(ConfigureRestClient) overload for delegate-based configuration
• Package Config: Configure project file to include README.md in NuGet package distribution