fix: resolve forwarded headers section

[PANiXiDA]

What changed

• AddHttp can now receive the root application IConfiguration.
• The package resolves the standard ForwardedHeaders section internally when it is present.
• Existing callers that pass a custom forwarded headers section still work.
• README examples now use builder.Services.AddHttp(builder.Configuration).

Why

• Consumers should not need to call configuration.GetSection(ForwardedHeaders) at every application entry point.
• Forwarded headers configuration stays centralized in the package.

How to verify locally

• dotnet restore
• dotnet format
• dotnet build --configuration Release
• dotnet test --configuration Release

Migrations / contracts / config / breaking changes

• No migrations.
• No breaking public API change.
• Configuration remains compatible with both root IConfiguration and an explicitly selected forwarded headers section.