[mongoose] Add useUnifiedTopology to ConnectionOptions (#41329)

* Add useUnifiedTopology to ConnectionOptions

useUnifiedTopology is the flag for using the new Server Discovery and Monitoring engine instead of current (deprecated) one.

* Update the tests to include useUnifiedTopology

types/mongoose/index.d.ts

@@ -371,6 +371,8 @@ declare module "mongoose" {
     useNewUrlParser?: boolean;
     /** Set to false to make findOneAndUpdate() and findOneAndRemove() use native findOneAndUpdate() rather than findAndModify(). */
     useFindAndModify?: boolean;
+    /** Flag for using new Server Discovery and Monitoring engine instead of current (deprecated) one */
+    useUnifiedTopology?: boolean;
 
     // Legacy properties - passed to the connection server instance(s)
     mongos?: any;

types/mongoose/mongoose-tests.ts

@@ -27,6 +27,7 @@ const connection2: Promise<mongoose.Mongoose> = mongoose.connect(connectUri, {
   bufferCommands: false,
   useNewUrlParser: true,
   useFindAndModify: true,
+  useUnifiedTopology: true,
   useCreateIndex: true,
   autoIndex: true,
   autoCreate: true,