Move WithCancellation/ConfigureAwait extension methods to TaskAsyncEnumerableExtensions

[joperezr]

cc: @stephentoub @terrajobst

Responding to changes made by dotnet/corefx#37367 where the extnsion methods where moved to a new type. This is the first time I make a change like this so please do let me know if I missed anything.

[terrajobst]

This is the first time I make a change like this so please do let me know if I missed anything.

I usually set the netstandard-api label for API related changes in .NET Standard and I also set a milestone. But this looks good as is.

@dotnet/nsboard This a change we need to take in order to react to an API change that we needed to make in CoreFX where IAsyncEnumerable<T> came from. I've removed the board because the API was previously signed off already.

[jskeet]

So just to be clear, this isn't a change from netstandard2.0 - it's changing methods/classes that have been added since netstandard2.0?

[stephentoub]

it's changing methods/classes that have been added since netstandard2.0?

Correct. These new methods were added in netstandard2.1, just to an existing type, and they're being moved to a new type instead.

[jskeet]

Not entirely sure about the new class name, but my understanding is that this PR isn't really deciding that - it's just reflecting a decision made elsewhere, which I don't want to try to relitigate :)