[SPEC] Add rewrite query parameter to the indices.validate_query API

rest-api-spec/api/indices.validate_query.json

@@ -66,6 +66,10 @@
         "lowercase_expanded_terms": {
           "type" : "boolean",
           "description" : "Specify whether query terms should be lowercased"
+        },
+        "rewrite": {
+          "type": "boolean",
+          "description": "Provide a more detailed explanation showing the actual Lucene query that will be executed."
         }
       }
     },

rest-api-spec/test/indices.validate_query/10_basic.yaml

@@ -33,4 +33,13 @@
   - match: {_shards.failed: 0}
   - match: {explanations.0.index: 'testing'}
   - match: {explanations.0.explanation: '*:*'}
+  
+  - do:
+      indices.validate_query:
+        rewrite: true
+
+  - is_true: valid
+  - match: {_shards.failed: 0}
+  - match: {explanations.0.index: 'testing'}
+  - match: {explanations.0.explanation: 'ConstantScore(*:*)'}