SQL: Basic REST spec and tests (elastic/x-pack-elasticsearch#3128)

Adds a basic REST spec and tests for the SQL and translate endpoints so
that clients can can execute these endpoints. We'll keep our complex
REST testing in Java REST tests though.

relates elastic/x-pack-elasticsearch#3115

Original commit: elastic/x-pack-elasticsearch@c5de301f3d

plugin/src/test/resources/rest-api-spec/api/xpack.sql.json

@@ -0,0 +1,16 @@
+{
+    "xpack.sql": {
+      "documentation": "Execute SQL",
+      "methods": [ "POST", "GET" ],
+      "url": {
+        "path": "/_sql",
+        "paths": [ "/_sql" ],
+        "parts": {},
+        "params": {}
+      },
+      "body": {
+        "description" : "Use the `query` element to start a query. Use the `cursor` element to continue a query.",
+        "required" : true
+      }
+    }
+  }

plugin/src/test/resources/rest-api-spec/api/xpack.sql.translate.json

@@ -0,0 +1,16 @@
+{
+    "xpack.sql.translate": {
+      "documentation": "Translate SQL into Elasticsearch queries",
+      "methods": [ "POST", "GET" ],
+      "url": {
+        "path": "/_sql/translate",
+        "paths": [ "/_sql/translate" ],
+        "parts": {},
+        "params": {}
+      },
+      "body": {
+        "description" : "Specify the query in the `query` element.",
+        "required" : true
+      }
+    }
+  }

plugin/src/test/resources/rest-api-spec/test/sql/sql.yml

@@ -0,0 +1,29 @@
+---
+"Execute some SQL":
+  - do:
+      bulk:
+        refresh: true
+        body:
+          - index:
+              _index: test
+              _type:  doc
+              _id:    1
+          - str: test1
+            int: 1
+          - index:
+              _index: test
+              _type:  doc
+              _id:    2
+          - str: test2
+            int: 2
+
+  - do:
+      xpack.sql:
+        body:
+          query: "SELECT * FROM test ORDER BY int asc"
+  - match: { columns.0.name: int }
+  - match: { columns.1.name: str }
+  - match: { rows.0.0: 1 }
+  - match: { rows.0.1: test1 }
+  - match: { rows.1.0: 2 }
+  - match: { rows.1.1: test2 }

plugin/src/test/resources/rest-api-spec/test/sql/translate.yml

@@ -0,0 +1,29 @@
+---
+"Translate SQL":
+  - do:
+      bulk:
+        refresh: true
+        body:
+          - index:
+              _index: test
+              _type:  doc
+              _id:    1
+          - str: test1
+            int: 1
+
+  - do:
+      xpack.sql.translate:
+        body:
+          query: "SELECT * FROM test ORDER BY int asc"
+  - match:
+      $body:
+        size: 1000
+        _source:
+          includes:
+            - str
+          excludes: []
+        docvalue_fields:
+          - int
+        sort:
+          - int:
+              order: asc