As discussed in #2180 I would like to add a new TakeUntil operator that takes a CancellationToken as parameter.

I think the best approach is to implement it as close as possible to the other TakeUntil operators, especially the one that takes a DateTimeOffset.

It should forward all notifications from the source to new observers and register the OnComplete Method on the prodived CancellationToken.
If the CancellationToken is already cancled, it should never subscibe to the source and call the OnComplete Method directly.

I would be happy to implement it.