@types/mongodb - add appname to MongoClientOptions (#23060)

- Based on
  https://mongodb.github.io/node-mongodb-native/3.0/api/MongoClient.html#.connect

types/mongodb/index.d.ts

@@ -92,6 +92,8 @@ export interface MongoClientOptions extends
     logger?: Object;
     // Default: false;
     validateOptions?: Object;
+    // The name of the application that created this MongoClient instance. 
+    appname?: string;
 }
 
 export interface SSLOptions {