Use API Guardian annotations.

Change-Id: Icfd30e9b06e5c7ee9f51c6879bfe4978a6f44600
diff --git a/pom.xml b/pom.xml
index 7d4ce60..1623195 100644
--- a/pom.xml
+++ b/pom.xml
@@ -58,6 +58,7 @@
     <spotless-plugin.version>2.17.6</spotless-plugin.version>
     <surefire-plugin.version>3.0.0-M5</surefire-plugin.version>
 
+    <apiguardian.version>1.1.2</apiguardian.version>
     <errorprone.version>2.10.0</errorprone.version>
     <google-java-format.version>1.13.0</google-java-format.version>
     <jetbrains-annotations.version>22.0.0</jetbrains-annotations.version>
@@ -94,6 +95,12 @@
       <version>${jetbrains-annotations.version}</version>
       <optional>true</optional>
     </dependency>
+    <dependency>
+      <groupId>org.apiguardian</groupId>
+      <artifactId>apiguardian-api</artifactId>
+      <version>${apiguardian.version}</version>
+      <optional>true</optional>
+    </dependency>
 
     <!-- Testing -->
     <dependency>