Add AddAppConfigurations extension for config-section-based client construction

[m-nash]

Summary

Adds two new AddAppConfigurations extension methods on IConfigurationBuilder that allow constructing an Azure App Configuration provider directly from an IConfiguration section using the new ConfigurationClientSettings from Azure.Data.AppConfiguration 1.9.0 and GetAzureClientSettings<T> from Azure.Identity 1.18.0.

This replaces the approach taken in Azure/azure-sdk-for-net#56619 (which created a separate package) by implementing the functionality directly in the existing provider package.

Resolves Azure/azure-sdk-for-net#55509

Changes

New APIs (marked [Experimental("SCME0002")])

• AddAppConfigurations(IConfigurationBuilder, string, bool) - Adds App Configuration using a named configuration section
• AddAppConfigurations(IConfigurationBuilder, string, Action<AzureAppConfigurationOptions>, bool) - Same with additional options callback

Package Updates

• Azure.Data.AppConfiguration 1.8.0 -> 1.9.0 (provides ConfigurationClientSettings)
• Azure.Identity 1.15.0 -> 1.18.0 (provides GetAzureClientSettings<T>)
• Microsoft.Extensions.DependencyInjection.Abstractions 8.0.2 -> 10.0.3 (required by transitive deps)

New Files

• ExperimentalAttribute.cs - Polyfill for [Experimental] attribute on netstandard2.0/2.1
• docs/ExperimentalFeatures.md - Documents the SCME0002 diagnostic ID usage
• ConfigurationSettingsTests.cs - 10 unit tests (6 validation + 4 happy-path)

Example Usage

var config = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json")
    .AddAppConfigurations("AppConfiguration")
    .Build();