shield: add rest API spec for authenticate api

Original commit: elastic/x-pack-elasticsearch@3dcfd5549b

elasticsearch/x-pack/shield/src/test/resources/rest-api-spec/api/shield.authenticate.json

@@ -0,0 +1,13 @@
+{
+  "shield.authenticate": {
+    "documentation": "Retrieve details about the currently authenticated user",
+    "methods": [ "GET" ],
+    "url": {
+      "path": "/_shield/authenticate",
+      "paths": [ "/_shield/authenticate" ],
+      "parts": {},
+      "params": {}
+    },
+    "body": null
+  }
+}

elasticsearch/x-pack/shield/src/test/resources/rest-api-spec/test/authenticate/10_basic.yaml

@@ -0,0 +1,11 @@
+---
+"Test authenticate api":
+
+  - do:
+      cluster.health:
+          wait_for_status: yellow
+  - do:
+      shield.authenticate: {}
+
+  - match: { username: "test_user" }
+  - match: { roles.0:  "admin" }