GenAPI is missing features that dotnet/runtime depends on

[ViktorHofer]

To consume the new Roslyn based GenAPI tooling, the following features that already exist in the legacy CCI based GenAPI need to be implemented:

• FollowTypeForwards
• LangVersion (?)
• ConditionalTypeList and DefaultCondition
• Fully qualified type names

In addition to that, there are a number of defects that would also need be fixed first. See the area filter: https://github.com/dotnet/sdk/issues?q=is%3Aopen+is%3Aissue+label%3AArea-GenAPI

cc @carlossanlop @ericstj @tannergooding @andriipatsula

[ViktorHofer]

dotnet/runtime won't use the new GenAPI in the .NET 8 timeframe. Moving to backlog.