Provide a data directory to plugins on demand

If a plugin requests a data directory with a @PluginData File
dependency, a data directory will be created automatically by the
server in $site_path/data/$plugin_name and passed to the plugin.

Plugins can use this to store any data they want.

Change-Id: I98de44a033889740cafc8a3d72c582c0e99b7ffd

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/annotations/PluginData.java

@@ -0,0 +1,42 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.annotations;
+
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import com.google.inject.BindingAnnotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+/**
+ * Local path where a plugin can store its own private data.
+ * <p>
+ * A plugin or extension may receive this string by Guice injection to discover
+ * a directory where it can store configuration or other data that is private:
+ *
+ * <pre>
+ * @Inject
+ * MyType(@PluginData java.io.File myDir) {
+ *   new FileInputStream(new File(myDir, &quot;my.config&quot;));
+ * }
+ * </pre>
+ */
+@Target({ElementType.PARAMETER, ElementType.FIELD})
+@Retention(RUNTIME)
+@BindingAnnotation
+public @interface PluginData {
+}