[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.

Start by reviewing the linked .NET MAUI pull request #3872 and the discussion in issue #4393 to understand the existing appsettings and hosting approaches. Define a concrete supported path for build-time settings injection and clear completion criteria before choosing an implementation.

Written by the indexing model from the issue text.