Refine validator and MVC validator configuration

Update `ValidationAutoConfiguration` and `WebMvcAutoConfiguration` to
ensure as much as possible that only a single Validator bean of each
type is registered.

Validation auto-configuration now does the following:
- If no validator is found: Registers a `LocalValidatorFactoryBean`
  (providing both Spring and JSR validation)
- If the user defines a Spring & JSR validator: Backs off
- If the user defines only a JSR validator: Adapts it to a Spring
  validator (without exposing another JSR implementation)

WebMvcAutoConfiguration auto-configuration has been updated to make
MVC validation follow common Spring Boot patterns:
- If not validator beans are found (due to the user excluding
  ValidationAutoConfiguration) a new `mvcValidator` bean will be
  registered.
- If a single validator bean is found it will be used for MVC
  validation.
- If multiple validator beans are defined it will either use the one
  named `mvcValidator` or it will register a new `mvcValidator` bean

Any automatically registered `mvcValidator` bean will not implement
the JSR validator interface.

Finally, it is no longer possible to provide an MVC validator via a
`WebMvcConfigurer`.

Fixes gh-8495

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/EndpointMvcIntegrationTests.java

@@ -41,6 +41,7 @@ import org.springframework.boot.actuate.endpoint.mvc.EndpointHandlerMappingCusto
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration;
 import org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration;
+import org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration;
@@ -156,9 +157,9 @@ public class EndpointMvcIntegrationTests {
 	@Documented
 	@Import({ EmbeddedServletContainerAutoConfiguration.class,
 			ServerPropertiesAutoConfiguration.class,
-			DispatcherServletAutoConfiguration.class, WebMvcAutoConfiguration.class,
-			JacksonAutoConfiguration.class, ErrorMvcAutoConfiguration.class,
-			PropertyPlaceholderAutoConfiguration.class })
+			DispatcherServletAutoConfiguration.class, ValidationAutoConfiguration.class,
+			WebMvcAutoConfiguration.class, JacksonAutoConfiguration.class,
+			ErrorMvcAutoConfiguration.class, PropertyPlaceholderAutoConfiguration.class })
 	protected @interface MinimalWebConfiguration {
 
 	}