Back port of Task Thin Execution List and List By Name

Updates api docs for tasks/thinexecutions/name and all HATEOAS links. Updates related tests for links.
Update find by name to return 404 when task definition doesn't exist.

Fixes #5995

Author: Corneil du Plessis

spring-cloud-dataflow-rest-client/src/main/java/org/springframework/cloud/dataflow/rest/client/TaskTemplate.java

@@ -146,42 +146,14 @@ public class TaskTemplate implements TaskOperations {
 		this.restTemplate = restTemplate;
 		this.dataFlowServerVersion = dataFlowServerVersion;
 
-		String version = VersionUtils.getThreePartVersion(dataFlowServerVersion);
-		if (VersionUtils.isDataFlowServerVersionGreaterThanOrEqualToRequiredVersion(version, VALIDATION_RELATION_VERSION)) {
-			Assert.isTrue(resources.getLink(VALIDATION_REL).isPresent(), ()->VALIDATION_REL + " relation is required");
-			this.validationLink = resources.getLink(VALIDATION_REL).get();
-		} else {
-			this.validationLink = null;
-		}
-
-		// TODO early 2.11.3-SNAPSHOT version didn't have this. Remove && resources.getLink(THIN_EXECUTIONS_RELATION).isPresent() when releasing 2.11.3
-		if(VersionUtils.isDataFlowServerVersionGreaterThanOrEqualToRequiredVersion(version, VALIDATION_THIN_TASK_VERSION) && resources.getLink(THIN_EXECUTIONS_RELATION).isPresent()) {
-			Assert.isTrue(resources.getLink(THIN_EXECUTIONS_RELATION).isPresent(), () -> THIN_EXECUTIONS_RELATION + " relation is required");
-			this.thinExecutionsLink = resources.getLink(THIN_EXECUTIONS_RELATION).get();
-		} else {
-			this.thinExecutionsLink = null;
-		}
-
-		if(resources.getLink(THIN_EXECUTIONS_BY_NAME_RELATION).isPresent()) {
-			this.thinExecutionsByNameLink = resources.getLink(THIN_EXECUTIONS_BY_NAME_RELATION).get();
-		} else {
-			this.thinExecutionsByNameLink = null;
-		}
-		if (VersionUtils.isDataFlowServerVersionGreaterThanOrEqualToRequiredVersion(version, VALIDATION_TASK_LAUNCH_VERSION)) {
-			Assert.isTrue(resources.getLink(EXECUTION_LAUNCH_RELATION).isPresent(), () -> EXECUTION_LAUNCH_RELATION + " relation is required");
-			this.executionLaunchLink = resources.getLink(EXECUTION_LAUNCH_RELATION).get();
-		} else {
-			this.executionLaunchLink = null;
-		}
-
-		if (VersionUtils.isDataFlowServerVersionGreaterThanOrEqualToRequiredVersion(version, EXECUTIONS_CURRENT_RELATION_VERSION)) {
-			Assert.isTrue(resources.getLink(EXECUTIONS_CURRENT_RELATION).isPresent(), ()-> EXECUTIONS_CURRENT_RELATION + " relation is required");
-			this.executionsCurrentLink = resources.getLink(EXECUTIONS_CURRENT_RELATION).get();
-		} else {
-			this.executionsCurrentLink = null;
-		}
+		this.validationLink = resources.getLink(VALIDATION_REL).get();
 		this.aboutLink = resources.getLink("about").get();
 
+		this.thinExecutionsLink = resources.getLink(THIN_EXECUTIONS_RELATION).orElse(null);
+		this.thinExecutionsByNameLink = resources.getLink(THIN_EXECUTIONS_BY_NAME_RELATION).orElse(null);
+		this.executionLaunchLink = resources.getLink(EXECUTION_LAUNCH_RELATION).orElse(null);
+		this.executionsCurrentLink = resources.getLink(EXECUTIONS_CURRENT_RELATION).orElse(null);
+
 		this.definitionsLink = resources.getLink(DEFINITIONS_RELATION).get();
 		this.definitionLink = resources.getLink(DEFINITION_RELATION).get();
 		this.executionsLink = resources.getLink(EXECUTIONS_RELATION).get();
@@ -305,14 +277,13 @@ public TaskExecutionThinResource.Page thinExecutionListByTaskName(String taskNam
 		if(thinExecutionsByNameLink != null) {
 			return restTemplate.getForObject(thinExecutionsByNameLink.expand(taskName).getHref(), TaskExecutionThinResource.Page.class);
 		} else {
-			return restTemplate.getForObject(executionsLink.expand(taskName).getHref(), TaskExecutionThinResource.Page.class);
+			return restTemplate.getForObject(executionByNameLink.expand(taskName).getHref(), TaskExecutionThinResource.Page.class);
 		}
 	}
 
 	@Override
 	public TaskExecutionResource.Page executionListByTaskName(String taskName) {
-		return restTemplate.getForObject(executionByNameLink.expand(taskName).getHref(),
-			TaskExecutionResource.Page.class);
+		return restTemplate.getForObject(executionByNameLink.expand(taskName).getHref(), TaskExecutionResource.Page.class);
 	}
 
 	@Override