[@types/mongodb] Added withTransaction definition and another necessary type. (#37085)

* [@types/mongodb] added withTransaction definition

* Added missing semicolon.

types/mongodb/index.d.ts

@@ -23,6 +23,7 @@
 //                 Erik Christensen <https://github.com/erikc5000>
 //                 Nick Zahn <https://github.com/Manc>
 //                 Jarom Loveridge <https://github.com/jloveridge>
+//                 Luis Pais <https://github.com/ranguna>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -126,6 +127,19 @@ export interface ClientSession extends EventEmitter {
      * Starts a new transaction with the given options.
      */
     startTransaction(options?: TransactionOptions): void;
+    
+    /**
+     * Runs a provided lambda within a transaction, retrying either the commit operation
+     * or entire transaction as needed (and when the error permits) to better ensure that
+     * the transaction can complete successfully.
+     *
+     * IMPORTANT: This method requires the user to return a Promise, all lambdas that do not
+     * return a Promise will result in undefined behavior.
+     * 
+     * @param fn Function to execute with the new session.
+     * @param options Optional settings for the transaction
+     */
+    withTransaction<T>(fn: WithTransactionCallback<T>, options?: TransactionOptions): Promise<T>;
 
 }
 
@@ -202,6 +216,8 @@ export interface MongoCallback<T> {
     (error: MongoError, result: T): void;
 }
 
+export type WithTransactionCallback<T> = (session: ClientSession) => Promise<T>;
+
 /** http://mongodb.github.io/node-mongodb-native/3.1/api/MongoError.html */
 export class MongoError extends Error {
     constructor(message: string);