[REST API] use a block literal for request bodies

rest-api-spec/src/main/resources/rest-api-spec/test/ingest/30_mutate.yaml

@@ -80,17 +80,18 @@
         type: test
         id: 1
         pipeline: "my_pipeline"
-        body: {
-          field_to_rename: "value",
-          field_to_remove: "old_value",
-          field_to_lowercase: "LOWERCASE",
-          field_to_uppercase: "uppercase",
-          field_to_trim: "   trimmed   ",
-          field_to_split: "127-0-0-1",
-          field_to_join: ["127","0","0","1"],
-          field_to_convert: ["127","0","0","1"],
-          field_to_gsub: "127-0-0-1"
-        }
+        body:  >
+          {
+            field_to_rename: "value",
+            field_to_remove: "old_value",
+            field_to_lowercase: "LOWERCASE",
+            field_to_uppercase: "uppercase",
+            field_to_trim: "   trimmed   ",
+            field_to_split: "127-0-0-1",
+            field_to_join: ["127","0","0","1"],
+            field_to_convert: ["127","0","0","1"],
+            field_to_gsub: "127-0-0-1"
+          }
 
   - do:
       get: