[Spring] Adds feature useOptional (#5968)

* Adds Valid annotation for request body (#4847)

If useBeanValidation is active, this change will add Valid annotation to ReqeustBody

* Adds generated samples for bean vaildation in spring boot (#4847)

* Adds feature option (#3819)

When you enable the feature useOptional the JavaSpring generator will use type Optional for non required parameters.

* Adds generated sample for #3819

* Adds generated sample for #3819

* Reverts commit for bean validation

* Adds generated sample for #3819

* Reverts commit for bean validation

* Fix alignment

* update spring samples

* update pom.xml to include spring use optional samples

* update artifactId to "spring-boot-beanvalidation"

* rpelace tab with 4-space

* check mvn task result

modules/swagger-codegen/src/main/resources/JavaSpring/api.mustache

@@ -23,6 +23,9 @@ import org.springframework.web.bind.annotation.RequestPart;
 import org.springframework.web.multipart.MultipartFile;
 
 import java.util.List;
+{{#useOptional}}
+import java.util.Optional;
+{{/useOptional}}
 {{#async}}
 import java.util.concurrent.{{^jdk8}}Callable{{/jdk8}}{{#jdk8}}CompletableFuture{{/jdk8}};
 {{/async}}