Find a home for System.Linq.Async features that .NET 10 won't be supporting #2199
Description
As per dotnet/runtime#79782 most of the System.Linq.Async functionality which has, by historical accident, been implemented in this repository, is now moving into the main .NET class libraries.
However, there is some functionality, such as the ToObservable extension for IAsyncEnumerable, which won't be ported over, because that's really an Rx feature.
We need to identify the features of our System.Linq.Async that we want to continue to make available, and work out where to put them. We won't be keeping them where they are because we will be deprecating the existing NuGet Package. But it's not clear where they will go yet, and that needs to be decided.