Add Runnable overrides to ease use of Java 8 language features

Runnable and Callable are both @FunctionalInterfaces. The difference is
that Callable requires a return value whereas Runnable does not, so in
situations where we don't care about a return value, rather than having to
add an unnecessary 'return null;' at the end of the lambda, we can simply
use a non-returning Runnable instead.

Unfortunately, owing to legacy reasons, Runnable is not declared to throw
checked exceptions whereas Callable is, so in situations where checked
exceptions are thrown we still need to have a 'return null;' call at the
end.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172935400

java/google/registry/util/Retrier.java

@@ -119,7 +119,7 @@ public class Retrier implements Serializable {
   };
 
   /**
-   * Retries a unit of work in the face of transient errors.
+   * Retries a unit of work in the face of transient errors and returns the result.
    *
    * <p>Retrying is done a fixed number of times, with exponential backoff, if the exception that is
    * thrown is on a whitelist of retryable errors. If the error is not on the whitelist, or if the
@@ -143,8 +143,20 @@ public class Retrier implements Serializable {
         moreRetryableErrors);
   }
 
+  /** Retries a unit of work in the face of transient errors. */
+  @SafeVarargs
+  public final void callWithRetry(
+      VoidCallable callable,
+      Class<? extends Throwable> retryableError,
+      Class<? extends Throwable>... moreRetryableErrors) {
+    callWithRetry(
+        callable.asCallable(),
+        retryableError,
+        moreRetryableErrors);
+  }
+
   /**
-   * Retries a unit of work in the face of transient errors.
+   * Retries a unit of work in the face of transient errors and returns the result.
    *
    * <p>Retrying is done a fixed number of times, with exponential backoff, if the exception that is
    * thrown is on a whitelist of retryable errors. If the error is not on the whitelist, or if the