Implement support for IAsyncEnumerable in the .NET client

[analogrelay]

Epic #8796

Split from https://github.com/aspnet/SignalR/issues/3166

Support for IAsyncEnumerable on the client (returning it from HubConnection.StreamAsync()). This is just a little thorny because of the open question of .NET Standard support for IAsyncEnumerable

[BrennanConroy]

IAsyncEnumerable is now available. See dotnet/corefx#33104

• We should replace our implementation of the interface at https://github.com/aspnet/AspNetCore/blob/6699fd056e4bbf491397067e01c191223a20f3b0/src/SignalR/src/Microsoft.AspNetCore.SignalR.Core/Internal/AsyncEnumeratorAdapters.cs#L72

• And we should create a new client method for StreamAsync which returns IAsyncEnumerable

• We can also consider allowing IAsyncEnumerable to be accepted in hub methods for the new streaming from client to server feature

[BrennanConroy]

Looks like our builds aren't ready for C# 8 yet

Moving back to 3.0.0