Add a simple Jenkinsfile to act as a webhook trigger

This must be triggered after the master Pipeline run for evergreen has
completed, in order to ensure that the evergreen backend is properly notified of
the last successful run .

Fixes JENKINS-53538

Jenkinsfile

@@ -0,0 +1,27 @@
+pipeline {
+    agent any
+
+    triggers {
+        upstream 'Infra/evergreen/master'
+    }
+
+    stages {
+        stage('Notify webhook') {
+            environment {
+                TOKEN = credentials('evergreen-upload-token')
+            }
+
+            steps {
+                echo 'Simulating a GitHub webhook call to the evergreen-upload Azure Function'
+                sh '''
+                    curl -v --fail -H 'X-GitHub-Event: push' \
+                        -d '{"ref":"refs/heads/master", "repository": {"full_name" : "jenkins-infra/evergreen"}, "commits": []}' \
+                        https://jenkins-infra-functions.azurewebsites.net/api/evergreen-upload?code=${TOKEN}&clientId=default
+                '''
+                echo '..webhook notified'
+            }
+        }
+    }
+}
+
+// vim: ft=groovy