Add support for Step Summary (#1642)

* First prototype of step summary environment variable

* Fix file contention issue

* Try to simplify cleaning up file references

* use step id as md file name, queue file attachment

* separate logic into attachment summary func

* Fix indentation

* Add (experimental) feature flag support

* reorganize summary upload determination logic

* file i/o exception handling + pr feedback

* Revert changes for now to reintroduce them later

* Add skeleton SetStepSummaryCommand

* Update step summary feature flag name

* Port ShouldUploadAttachment from previous iteration

* Port QueueStepSummaryUpload from previous iteration

* Improve exception handling when uploading attachment

* Add some minor logging improvements

* Refuse to upload files larger than 128k

* Implement secrets scrubbing

* Add TODO comment to remove debugging temp files

* Add first tests

* Add test for secret masking

* Add some naming/style fixes suggested in feedback

* inline check for feature flag

* Inline method for style consistency

* Make sure that scrubbed file doesn't exist before creating it

* Rename SetStepSummaryCommand to CreateStepSummaryCommand

* Fix error handling messages

* Fix file command name when registering extension

* Remove unnecessary file deletion

Co-authored-by: Rob Herley <robherley@github.com>

src/Sdk/DTWebApi/WebApi/TaskAttachment.cs

@@ -102,4 +102,10 @@ namespace GitHub.DistributedTask.WebApi
         public static readonly String FileAttachment = "DistributedTask.Core.FileAttachment";
         public static readonly String DiagnosticLog = "DistributedTask.Core.DiagnosticLog";
     }
+
+    [GenerateAllConstants]
+    public class ChecksAttachmentType
+    {
+        public static readonly String StepSummary = "Checks.Step.Summary";
+    }
 }