Add FilePattern support for .github\workflow sync (#1856)

Given we cannot create subfolders under .github\workflow folder
and we have repos that want additional workflows then we have
in the tools repo we need to setup a file pattern matching for
the sync processing to only sync the ones we want to be common.

For now just use the existing *event* pattern but we can move them
to another pattern similar to *common* or *sync* in the future as
we add more common workflows.

Co-authored-by: Wes Haggard <Wes.Haggard@microsoft.com>

eng/common/pipelines/templates/steps/eng-common-workflow-enforcer.yml

@@ -21,7 +21,7 @@ steps:
         }
         if ((!"$(System.PullRequest.SourceBranch)".StartsWith("sync-.github/workflows")) -and "$(System.PullRequest.TargetBranch)" -match "^(refs/heads/)?$(DefaultBranch)$")
         {
-          $filesInCommonDir  = & "eng/common/scripts/get-changedfiles.ps1" -DiffPath '.github/workflows/*' -DiffFilterType ""
+          $filesInCommonDir  = & "eng/common/scripts/get-changedfiles.ps1" -DiffPath '.github/workflows/*event*' -DiffFilterType ""
           if (($LASTEXITCODE -eq 0) -and ($filesInCommonDir.Count -gt 0))
           {
             Write-Host "##vso[task.LogIssue type=error;]Changes to files under '.github/workflows' directory should not be made in this Repo`n${filesInCommonDir}"