Add AppSettings source generator for IConfigurationBuilder extension

[Copilot]

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This PR introduces a new source generator that automatically creates an extension method for IConfigurationBuilder to load configuration values from an appsettings.json file at build time.

What does this solve?

Developers often need to include configuration data from JSON files in their MAUI applications. Previously, this required manually reading and parsing JSON files at runtime. This source generator automates the process by generating compile-time code that embeds the configuration values directly.

How it works

When you have an appsettings.json file in your project root:

{
  "AppName": "My MAUI App",
  "Settings": {
    "EnableLogging": true,
    "MaxConnections": 100
  },
  "ConnectionString": "Server=localhost;Database=MyDB",
  "Features": ["Feature1", "Feature2"]
}

The source generator automatically creates an extension method that you can use like this:

var builder = MauiApp.CreateBuilder();
builder.Configuration.AddLocalAppSettings();
var app = builder.Build();

var configuration = app.Services.GetRequiredService<IConfiguration>();
Console.WriteLine(configuration["AppName"]); // "My MAUI App"
Console.WriteLine(configuration["Settings:EnableLogging"]); // "true"
Console.WriteLine(configuration["Features:0"]); // "Feature1"

Generated code

The source generator creates an extension method with hardcoded values from your JSON:

public static IConfigurationBuilder AddLocalAppSettings(this IConfigurationBuilder configurationBuilder)
{
    var configurationData = new Dictionary<string, string>
    {
        { "AppName", "My MAUI App" },
        { "Settings:EnableLogging", "true" },
        { "Settings:MaxConnections", "100" },
        { "ConnectionString", "Server=localhost;Database=MyDB" },
        { "Features:0", "Feature1" },
        { "Features:1", "Feature2" },
    };
    return configurationBuilder.AddInMemoryCollection(configurationData);
}

Key features

• Build-time generation: No runtime JSON parsing overhead
• Flattened keys: Supports nested objects and arrays using standard configuration key format (Settings:EnableLogging, Features:0)
• Type preservation: Numbers, booleans, and strings are properly converted
• Incremental compilation: Only regenerates when appsettings.json changes
• Zero dependencies: Uses only built-in .NET configuration APIs

Implementation details

• Source generator: Microsoft.Maui.Core.ConfigurationSourceGen using IIncrementalGenerator
• Integration: Included as analyzer in Microsoft.Maui.Core package
• Distribution: Automatically available to all MAUI applications
• Testing: Comprehensive unit tests covering JSON parsing, code generation, and edge cases

The source generator integrates seamlessly with the existing MAUI build process and requires no additional setup - just add an appsettings.json file to your project and use builder.Configuration.AddLocalAppSettings().

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[jfversluis]

/azp run MAUI-public

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Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Cool idea, lets revisit later