✨ Support multiple projects with automatic dependency registration

[matt-goldman]

Is your feature request related to a problem? Please describe.
The docs currently state that the automatic dependency registration source generator only works with single-project solutions.

Describe the solution you'd like
Support automatic dependency resolution in class libraries.

Describe alternatives you've considered
Attempted to make this work but scanning dependencies included all assemblies, including .NET MAUI controls, etc.

Additional context
It may be the case that this actually already works - you could simply call .UseAutoDependencies in your class library - if the class library is using the generic host builder for dependency injection into the main project. This needs to be tested.