build: Add default constructor and javadoc for metrics, nat, pki, plugins, services, testutil, util (#7071)

Signed-off-by: Usman Saleem <usman@usmans.info>

testutil/src/main/java/org/hyperledger/besu/kvstore/AbstractKeyValueStorageTest.java

@@ -40,6 +40,8 @@ import org.junit.jupiter.api.Test;
 /** The Abstract key value storage test. */
 @Disabled
 public abstract class AbstractKeyValueStorageTest {
+  /** Default Constructor */
+  protected AbstractKeyValueStorageTest() {}
 
   /**
    * Create store key value storage.

testutil/src/main/java/org/hyperledger/besu/testutil/MockExecutorService.java

@@ -36,6 +36,14 @@ public class MockExecutorService implements ExecutorService {
 
   private final List<ExecutorTask<?>> tasks = new ArrayList<>();
 
+  /** Default constructor */
+  public MockExecutorService() {}
+
+  /**
+   * Gets tasks.
+   *
+   * @return the tasks
+   */
   /**
    * Gets futures.
    *

testutil/src/main/java/org/hyperledger/besu/testutil/MockScheduledExecutor.java

@@ -25,6 +25,8 @@ import java.util.concurrent.TimeoutException;
 
 /** The mock scheduled executor */
 public class MockScheduledExecutor extends MockExecutorService implements ScheduledExecutorService {
+  /** Default constructor */
+  public MockScheduledExecutor() {}
 
   @Override
   public ScheduledFuture<?> schedule(

testutil/src/main/java/org/hyperledger/enclave/testutil/EnclaveKeyUtils.java

@@ -34,6 +34,9 @@ import org.slf4j.LoggerFactory;
 public class EnclaveKeyUtils {
   private static final Logger LOG = LoggerFactory.getLogger(EnclaveKeyUtils.class);
 
+  /** Default constructor */
+  private EnclaveKeyUtils() {}
+
   /**
    * Utility method to load the enclave public key. Possible input values are the names of the *.pub
    * files in the resources folder.

testutil/src/main/java/org/hyperledger/enclave/testutil/TesseraTestHarnessFactory.java

@@ -30,6 +30,9 @@ import org.testcontainers.containers.Network;
 public class TesseraTestHarnessFactory {
   private static final String storage = "memory";
 
+  /** Default constructor */
+  private TesseraTestHarnessFactory() {}
+
   /**
    * Create tessera test harness.
    *