This change is meant to allow cancellation of Helix Jobs asynchronously by different pipelines / stages, because the existing model (cancellation on build cancellation) does not currently work when the build stage waiting on the helix jobs times out, which is unfortunately the top reason this happens.

Addresses #7113

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

• I don't see any reasonable way to add tests for this, but I exercised authenticated and non-authenticated scenarios locally using unit tests by adding steps like this to UnitTests.proj:

  <Target Name="WaitThirtySecondsAndCancelThoseJobs" AfterTargets="CoreTest">
    <Message Importance="High" Text="Wasting 30 seconds then cancelling the job" />
    <Exec Command="ping -n 30 127.0.0.1 > nul" />
    <CancelHelixJobs Jobs="@(SentJob)" AccessToken="$(HelixAccessToken)" />
    <Message Importance="High" Text="did it work?" />
  </Target>