client: add PMD source code analyzer

This catches common bugs, dead code, etc., and also performs some more advanced
code style checks compared to checkstyle.

Co-authored-by: pilino1234 <pilino@posteo.de>

ci/settings.xml

@@ -259,6 +259,7 @@ under the License.
       <properties>
         <maven.javadoc.skip>false</maven.javadoc.skip>
         <checkstyle.skip>false</checkstyle.skip>
+        <pmd.skip>false</pmd.skip>
       </properties>
     </profile>
   </profiles>