Add server:capabilities method (#693)

## What does this PR do ?

add the following action `server:capabilities`

Author: clementbolin

index.ts

@@ -31,3 +31,4 @@ export * from './src/controllers/Collection';
 export * from './src/controllers/Document';
 export * from './src/controllers/Index';
 export * from './src/controllers/Realtime';
+export * from './src/controllers/Server';

src/Kuzzle.ts

@@ -96,7 +96,7 @@ export class Kuzzle extends KuzzleEventEmitter {
   public ms: any;
   public realtime: RealtimeController;
   public security: SecurityController;
-  public server: any;
+  public server: ServerController;
 
   private _protectedEvents: any;
   private _offlineQueue: any;

src/controllers/Server.ts

@@ -27,6 +27,18 @@ export class ServerController extends BaseController {
       .then(response => response.result.exists);
   }
 
+  /**
+   * Returns the Kuzzle capabilities
+   * @param {Object} options - {queuable: Boolean(true)}
+   * @example https://docs.kuzzle.io/core/2/api/controllers/server/capabilities/#response
+   * @returns {Promise<Object>}
+   */
+  capabilities (options: ArgsServerControllerGetAllStats) {
+    return this.query({
+      action: 'capabilities'
+    }, options)
+      .then(response => response.result);
+  }
 
   /**
    * Returns all stored statistics frames

test/controllers/server.test.js

@@ -113,6 +113,59 @@ describe('Server Controller', () => {
     });
   });
 
+  describe('#capabilities', () => {
+    const
+      expectQuery = {
+        controller: 'server',
+        action: 'capabilities'
+      },
+      result = {
+        limits: {
+          concurrentRequests: 100,
+          documentsFetchCount: 10000,
+          documentsWriteCount: 200,
+          loginsPerSecond: 50,
+          requestsBufferSize: 50000,
+          requestsBufferWarningThreshold: 5000,
+          subscriptionConditionsCount: 100,
+          subscriptionRooms: 1000000,
+          subscriptionDocumentTTL: 259200000
+        },
+        plugins: {},
+        version: '<current kuzzle version>',
+      };
+    
+    it('should call query with the right arguments and return Promise which resolves the capabilities', async () => {
+      kuzzle.query.resolves({ result });
+
+      let res = await kuzzle.server.capabilities();
+
+      should(kuzzle.query).be.calledOnce();
+      should(kuzzle.query).be.calledWith(expectQuery, {});
+      should(res).match(result);
+
+      kuzzle.query.resetHistory();
+      res = await kuzzle.server.capabilities({queuable: false});
+
+      should(kuzzle.query).be.calledOnce();
+      should(kuzzle.query).be.calledWith(expectQuery, {queuable: false});
+      should(res).match(result);
+    });
+
+    it('should reject the promise if an error occurs', async () => {
+      const error = new Error('foobar error');
+      error.status = 412;
+      kuzzle.query.rejects(error);
+
+      try {
+        await kuzzle.server.capabilities();
+      }
+      catch (err) {
+        should(err).be.match({status: 412, message: 'foobar error'});
+      }
+    });
+  });
+
   describe('#getConfig', () => {
     const
       expectedQuery = {