Query: Add ForEachAsync operator with async lambda

[smitpatel]

From #11861

ForEachAsync takes sync lambda. We can add overload which takes async lambda too so async func can be called from it.

[divega]

We should get out of the business of writing extension methods and extending IX Async when IAsyncEnumerable support in C# 8 is happening soon. But this should be a nice to have for that work.

[tuespetre]

soon

Ohhh? Do you know some knowledge about some things @divega?

[divega]

@tuespetre really nothing that isn’t public knowledge: dotnet/csharplang#43 is marked as 8.0 candidate, and we have been discussing it for a long time now.

[tuespetre]

Dang, I was hoping your “soon” was sooner than the “soon” I’ve been telling myself for a while. 🤐

[divega]

Closing as duplicate of #12425.