Add "onClass" condition on web server auto-configurations

This commit adds new Conditional annotations on auto-configurations
creating the Servlet/Reactive web servers. These conditions only depend
on request primitives used by the servers themselves and aren't
framework specific.

Closes gh-10163

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/ReactiveWebServerAutoConfiguration.java

@@ -24,6 +24,7 @@ import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.boot.autoconfigure.AutoConfigureOrder;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
 import org.springframework.boot.autoconfigure.web.ServerProperties;
@@ -35,6 +36,7 @@ import org.springframework.context.annotation.Import;
 import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
 import org.springframework.core.Ordered;
 import org.springframework.core.type.AnnotationMetadata;
+import org.springframework.http.ReactiveHttpInputMessage;
 import org.springframework.util.ObjectUtils;
 
 /**
@@ -45,6 +47,7 @@ import org.springframework.util.ObjectUtils;
  */
 @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
 @Configuration
+@ConditionalOnClass(ReactiveHttpInputMessage.class)
 @ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
 @EnableConfigurationProperties(ServerProperties.class)
 @Import({ ReactiveWebServerAutoConfiguration.BeanPostProcessorsRegistrar.class,

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/ServletWebServerFactoryAutoConfiguration.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure.web.servlet;
 
 import javax.servlet.Servlet;
+import javax.servlet.ServletRequest;
 
 import io.undertow.Undertow;
 import org.apache.catalina.startup.Tomcat;
@@ -66,6 +67,7 @@ import org.springframework.util.ObjectUtils;
  */
 @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
 @Configuration
+@ConditionalOnClass(ServletRequest.class)
 @ConditionalOnWebApplication(type = Type.SERVLET)
 @EnableConfigurationProperties(ServerProperties.class)
 @Import(BeanPostProcessorsRegistrar.class)