[1742] Ensure multiple composite annoations are correctly written. (#2311)

* [1742] Ensure multiple composite annoations are correctly written.

This implementation uses a collector pattern to allow embedded ExecutionContexts to stash Issue objects for later processing by a non-embedded ancestor ExecutionContext.

Also:
 - Provide explicit constructor implementations for ExecutionContext
 - Leverage explicit constructors to solidify immutability of several ExecutionContext class members.
 - Fixed erroneous call to ExecutionContext.Complete in CompositeActionHandler.cs
 - Use a consistent timestamp for FinishTime in ExecutionContext::Complete

* Ensure collected issues are processed only by a non-embedded ExecutionContext.
This was already implicit.  Now, just making it explicit.

* Provide a clear mechanism that allows callers to opt-in/opt-out of ExecutionContext::AddIssue's logging behavior.
* Addressed deserialization inconsistencies in TimelineRecord.cs
* Added TimelineRecord unit tests.
* Refined unit tests related to TimelineRecord::Variables case-insensitivity
* Add a unit test that verifies ExecutionContextLogOptions::LogMessageOverride has the desired effect.
* Responded to PR feedback.
* Don't allow embedded ExecutionContexts to add Issues to a TimelineRecord

src/Runner.Worker/JobRunner.cs

@@ -120,7 +120,8 @@ namespace GitHub.Runner.Worker
                         default:
                             throw new ArgumentException(HostContext.RunnerShutdownReason.ToString(), nameof(HostContext.RunnerShutdownReason));
                     }
-                    jobContext.AddIssue(new Issue() { Type = IssueType.Error, Message = errorMessage });
+                    var issue = new Issue() { Type = IssueType.Error, Message = errorMessage };
+                    jobContext.AddIssue(issue, ExecutionContextLogOptions.Default);
                 });
 
                 // Validate directory permissions.