build: add adk.build.directory property to relocate build output

Introduce an adk.build.directory property (default ${project.basedir}/target)
used as the build output directory, so the location can be relocated with
-Dadk.build.directory=... without changing the default.

PiperOrigin-RevId: 962178122

Author: damianmomotgoogle

pom.xml

@@ -42,6 +42,9 @@
     <java.version>17</java.version>
     <maven.compiler.release>${java.version}</maven.compiler.release>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <!-- Build output root; override to relocate build output (e.g. onto local
+         disk when the checkout is on a slow filesystem). -->
+    <adk.build.directory>${project.basedir}/target</adk.build.directory>
     <maven.checkstyle.plugin.version>3.6.0</maven.checkstyle.plugin.version>
 
     <auto-value.version>1.11.1</auto-value.version>
@@ -299,6 +302,7 @@
         <version>2.2.4</version>
       </extension>
     </extensions>
+    <directory>${adk.build.directory}</directory>
     <pluginManagement>
       <plugins>
         <plugin>