Allow plugins to get their canonical web URL injected

This change allows a plugin to get its canonical web URL injected at
runtime.

The URL is composed of the server's canonical web URL and the plugin's
name, i.e. `http://review.example.com:8080/plugins/plugin-name/`.

The canonical web URL may be injected into any .jar plugin regardless
of whether or not the plugin provides an HTTP servlet.

Change-Id: Id1a8489d49bfebff4ffdb8de58e9d0445113a09b

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

@@ -0,0 +1,42 @@
+// Copyright (C) 2013 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;
+
+/**
+ * Annotation applied to a String containing the plugin canonical web URL.
+ * <p>
+ * A plugin or extension may receive this string by Guice injection to discover
+ * the canonical web URL under which the plugin is available:
+ *
+ * <pre>
+ *  @Inject
+ *  MyType(@PluginCanonicalWebUrl String myUrl) {
+ *  ...
+ *  }
+ * </pre>
+ */
+@Target({ElementType.PARAMETER, ElementType.FIELD})
+@Retention(RUNTIME)
+@BindingAnnotation
+public @interface PluginCanonicalWebUrl {
+}