Allow a project to describe how it should be referenced

Today, if you want to reference a code generator and use it, you need to do something like

```xml

```

in each referencing project.

But you know in the project that it's a source generator! It'd be great if you could guide the default consumption of the `ProjectReference` from within the project.

Roslyn has considered adding a `SourceGeneratorProjectReference` to simplify this (https://github.com/dotnet/roslyn/pull/66974) but it is complicated enough that it'd be better as an MSBuild feature.

Ideally a project would be able to influence

* `TargetFramework` negotiation
* `IsRidAgnostic`
* `OutputItemType`

and possibly other things.