[TEST] add 'yaml' feature for the test runner (#19436)

Also renamed 30_yaml.yaml to 30_json.yaml since it tests json, not yaml

rest-api-spec/src/main/resources/rest-api-spec/test/cat.aliases/20_headers.yaml

@@ -1,7 +1,7 @@
 ---
 "Simple alias with yaml body through Accept header":
   - skip:
-      features: headers
+      features: ["headers", "yaml"]
 
   - do:
         indices.create:

rest-api-spec/src/main/resources/rest-api-spec/test/get/50_with_headers.yaml

@@ -1,7 +1,7 @@
 ---
 "REST test with headers":
   - skip:
-      features: headers
+      features: ["headers", "yaml"]
 
   - do:
       index:

test/framework/src/main/java/org/elasticsearch/test/rest/support/Features.java

@@ -34,7 +34,7 @@ import java.util.List;
  */
 public final class Features {
 
-    private static final List<String> SUPPORTED = Arrays.asList("stash_in_path", "groovy_scripting", "headers", "embedded_stash_key");
+    private static final List<String> SUPPORTED = Arrays.asList("stash_in_path", "groovy_scripting", "headers", "embedded_stash_key", "yaml");
 
     private Features() {