Redesign RetryPolicy to directly incorporate BackOff

After experimenting with our newly introduced core retry support
(RetryPolicy, RetryTemplate, etc.) and @⁠Retryable support, it
became apparent that there are overlapping concerns between the current
RetryPolicy and BackOff contracts.

- RetryPolicy and BackOff both have stateful executions: RetryExecution
  and BackOffExecution. However, only one stateful execution is
  necessary.

- FixedBackOff and ExponentialBackOff already incorporate "retry" logic
  in terms of max attempts, max elapsed time, etc. Thus, there is no
  need to duplicate such behavior in a RetryPolicy and its
  RetryExecution.

- RetryTemplate currently accepts both a RetryPolicy and a BackOff in
  order to instrument the retry algorithm. However, users would
  probably rather focus on configuring all "retry" logic via a single
  mechanism.

In light of the above, this commit directly incorporates BackOff
in RetryPolicy as follows.

- Remove the RetryExecution interface and move its shouldRetry() method
  to RetryPolicy, replacing the current RetryExecution start() method.

- Introduce a default getBackOff() method in the RetryPolicy interface.

- Introduce RetryPolicy.withDefaults() factory method.

- Completely overhaul the RetryPolicy.Builder to provide support for
  configuring a BackOff strategy.

- Remove BackOff configuration from RetryTemplate.

- Revise the method signatures of callbacks in RetryListener.

The collective result of these changes can be witnessed in the
reworked implementation of AbstractRetryInterceptor.

RetryPolicy retryPolicy = RetryPolicy.builder()
		.includes(spec.includes())
		.excludes(spec.excludes())
		.predicate(spec.predicate().forMethod(method))
		.maxAttempts(spec.maxAttempts())
		.delay(Duration.ofMillis(spec.delay()))
		.maxDelay(Duration.ofMillis(spec.maxDelay()))
		.jitter(Duration.ofMillis(spec.jitter()))
		.multiplier(spec.multiplier())
		.build();

RetryTemplate retryTemplate = new RetryTemplate(retryPolicy);

See gh-34716
See gh-34529
See gh-35058
Closes gh-35110

spring-aop/src/main/java/org/springframework/aop/retry/AbstractRetryInterceptor.java

@@ -34,7 +34,6 @@ import org.springframework.core.retry.RetryPolicy;
 import org.springframework.core.retry.RetryTemplate;
 import org.springframework.core.retry.Retryable;
 import org.springframework.util.ClassUtils;
-import org.springframework.util.backoff.ExponentialBackOff;
 
 /**
  * Abstract retry interceptor implementation, adapting a given
@@ -89,26 +88,17 @@ public abstract class AbstractRetryInterceptor implements MethodInterceptor {
 			}
 		}
 
-		RetryTemplate retryTemplate = new RetryTemplate();
-
-		RetryPolicy.Builder policyBuilder = RetryPolicy.builder();
-		for (Class<? extends Throwable> include : spec.includes()) {
-			policyBuilder.includes(include);
-		}
-		for (Class<? extends Throwable> exclude : spec.excludes()) {
-			policyBuilder.excludes(exclude);
-		}
-		policyBuilder.predicate(spec.predicate().forMethod(method));
-		policyBuilder.maxAttempts(spec.maxAttempts());
-		retryTemplate.setRetryPolicy(policyBuilder.build());
-
-		ExponentialBackOff backOff = new ExponentialBackOff();
-		backOff.setInitialInterval(spec.delay());
-		backOff.setJitter(spec.jitter());
-		backOff.setMultiplier(spec.multiplier());
-		backOff.setMaxInterval(spec.maxDelay());
-		backOff.setMaxAttempts(spec.maxAttempts());
-		retryTemplate.setBackOffPolicy(backOff);
+		RetryPolicy retryPolicy = RetryPolicy.builder()
+				.includes(spec.includes())
+				.excludes(spec.excludes())
+				.predicate(spec.predicate().forMethod(method))
+				.maxAttempts(spec.maxAttempts())
+				.delay(Duration.ofMillis(spec.delay()))
+				.maxDelay(Duration.ofMillis(spec.maxDelay()))
+				.jitter(Duration.ofMillis(spec.jitter()))
+				.multiplier(spec.multiplier())
+				.build();
+		RetryTemplate retryTemplate = new RetryTemplate(retryPolicy);
 
 		try {
 			return retryTemplate.execute(new Retryable<>() {