feat(cognito): Implement user pool and user pool client constructs (#1615)

This commit adds initial support for Cognito User Pools.
`UserPool` allows selecting the type of sign-in (username vs email, etc)
with options consistent with what is presented in the console.
`UserPool` also supports setting alias attributes & auto-verified attributes,
as well as setting Lambda function triggers.

A basic implementation of app clients is implemented in `UserPoolClient`.

Author: dotxlem

packages/@aws-cdk/aws-cognito/lib/index.ts

@@ -1,2 +1,5 @@
 // AWS::Cognito CloudFormation Resources:
 export * from './cognito.generated';
+
+export * from './user-pool';
+export * from './user-pool-client';

packages/@aws-cdk/aws-cognito/lib/user-pool-client.ts

@@ -0,0 +1,67 @@
+import cdk = require('@aws-cdk/cdk');
+import { CfnUserPoolClient } from './cognito.generated';
+import { IUserPool } from './user-pool';
+
+/**
+ * Types of authentication flow
+ */
+export enum AuthFlow {
+  /**
+   * Enable flow for server-side or admin authentication (no client app)
+   */
+  AdminNoSrp = 'ADMIN_NO_SRP_AUTH',
+
+  /**
+   * Enable custom authentication flow
+   */
+  CustomFlowOnly = 'CUSTOM_AUTH_FLOW_ONLY',
+
+  /**
+   * Enable auth using username & password
+   */
+  UserPassword = 'USER_PASSWORD_AUTH'
+}
+
+export interface UserPoolClientProps {
+  /**
+   * Name of the application client
+   * @default cloudformation generated name
+   */
+  clientName?: string;
+
+  /**
+   * The UserPool resource this client will have access to
+   */
+  userPool: IUserPool;
+
+  /**
+   * Whether to generate a client secret
+   * @default false
+   */
+  generateSecret?: boolean;
+
+  /**
+   * List of enabled authentication flows
+   * @default no enabled flows
+   */
+  enabledAuthFlows?: AuthFlow[]
+}
+
+/**
+ * Define a UserPool App Client
+ */
+export class UserPoolClient extends cdk.Construct {
+  public readonly clientId: string;
+
+  constructor(scope: cdk.Construct, id: string, props: UserPoolClientProps) {
+    super(scope, id);
+
+    const userPoolClient = new CfnUserPoolClient(this, 'Resource', {
+      clientName: props.clientName,
+      generateSecret: props.generateSecret,
+      userPoolId: props.userPool.userPoolId,
+      explicitAuthFlows: props.enabledAuthFlows
+    });
+    this.clientId = userPoolClient.userPoolClientId;
+  }
+}