Migrate Extensions 2.1 libraries to aspnetcore

Migrate packages from https://github.com/dotnet/extensions/tree/release/2.1 to aspnetcore's 2.1 branch. We can use https://github.com/dotnet/aspnetcore-internal/tree/main/scripts/consolidation to maintain the `git` history. the baseline files will need a manual fixup, but that should be the only "Special" work needed.