[Enhancement] Add appsettings support

Description

It would be useful to have a way for apps to build with different settings (ie: the appsettings.json file pattern already common with Microsoft.Extensions.*).

This is often done like in: https://github.com/dotnet/maui/pull/3872

There’s discussion around removing hosting for .NET 6 MAUI GA: https://github.com/dotnet/maui/issues/4393

In the future we should find a path forward to support this.

One idea is to use a source generator to create the code to inject values from the .json files at build time so that we do not pay a performance price for reading and parsing json files from embedded resources.

[jamesmontemagno]

Is possible today via -> https://montemagno.com/dotnet-maui-appsettings-json-configuration/ however some startup performance considerations with this method.

[Ghevi]

I don’t know if it’s already included, but can you also consider supporting IOptionsMonitor/IOptionsSnapshot? I have a scenario where I need to reload settings at runtime, I understand it’s something not many people have to do, but if possible it would be awesome. Is there a way to do this at the moment? The best way i found is:

   configuration["MyOptions"] = JsonSerializer.Serializer(new MyOptions { value = "new value" });
   (configuration as IConfigurationRoot).Reload();

and subscribe with the reload token of IConfiguration in the services where i need those options.