Add auth options into MongoClientOptions (#24934)

Reference: https://github.com/DefinitelyTyped/DefinitelyTyped/issues/24906#issuecomment-380486889

types/mongodb/index.d.ts

@@ -97,6 +97,10 @@ export interface MongoClientOptions extends
     validateOptions?: Object;
     // The name of the application that created this MongoClient instance.
     appname?: string;
+    auth?: {
+        user: string;
+        password: string;
+    }
 }
 
 export interface SSLOptions {