Add arch test for preview API usage

pom.xml

@@ -410,6 +410,12 @@
       <artifactId>commons-lang3</artifactId>
       <version>3.9</version>
     </dependency>
+    <dependency>
+      <groupId>com.tngtech.archunit</groupId>
+      <artifactId>archunit</artifactId>
+      <version>0.14.1</version>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.hamcrest</groupId>
       <artifactId>hamcrest</artifactId>

src/test/java/org/kohsuke/github/ArchTests.java

@@ -0,0 +1,58 @@
+package org.kohsuke.github;
+
+import com.tngtech.archunit.core.domain.JavaClasses;
+import com.tngtech.archunit.core.importer.ClassFileImporter;
+import com.tngtech.archunit.core.importer.ImportOption;
+import com.tngtech.archunit.lang.ArchRule;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.classes;
+import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.fields;
+import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.methods;
+import static org.junit.Assert.assertTrue;
+
+public class ArchTests {
+
+    private static final JavaClasses classFiles = new ClassFileImporter()
+            .withImportOption(new ImportOption.DoNotIncludeTests())
+            .withImportOption(new ImportOption.DoNotIncludeJars())
+            .importPackages("org.kohsuke.github");
+
+    @BeforeClass
+    public static void beforeClass() {
+        assertTrue(classFiles.size() > 0);
+    }
+
+    @Test
+    public void testPreviewsAreFlaggedAsDeprecated() {
+
+        String reason = "all preview APIs must be annotated as @Deprecated until they are promoted to stable";
+
+        ArchRule classRule = classes().that()
+                .areAnnotatedWith(Preview.class)
+                .should()
+                .beAnnotatedWith(Deprecated.class)
+                .because(reason);
+
+        ArchRule methodRule = methods().that()
+                .areAnnotatedWith(Preview.class)
+                .should()
+                .beAnnotatedWith(Deprecated.class)
+                .because(reason);
+
+        ArchRule enumFieldsRule = fields().that()
+                .areDeclaredInClassesThat()
+                .areEnums()
+                .and()
+                .areAnnotatedWith(Preview.class)
+                .should()
+                .beAnnotatedWith(Deprecated.class)
+                .because(reason);
+
+        classRule.check(classFiles);
+        enumFieldsRule.check(classFiles);
+        methodRule.check(classFiles);
+
+    }
+}