ci: Add exec-linkage-checker maven profile to run linkage checker (#2398)

Author: lqiu96

dependencies/pom.xml

@@ -121,6 +121,22 @@
   </dependencies>
 
   <profiles>
+    <profile>
+      <id>exec-linkage-checker</id>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>exec-maven-plugin</artifactId>
+            <version>3.5.0</version>
+            <configuration>
+              <skip>false</skip>
+              <mainClass>com.google.cloud.tools.opensource.classpath.LinkageCheckerMain</mainClass>
+            </configuration>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
     <profile>
       <id>java8-incompatible-reference-check</id>
       <build>