feat(cognito): user pool - case sensitivity for sign in

### Commit Message
feat(cognito): user pool - case sensitivity for sign in (aws#7988 

fixes aws#7235
### End Commit Message

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-cognito/README.md

@@ -148,6 +148,9 @@ new UserPool(this, 'myuserpool', {
 });
 ```
 
+A user pool can optionally ignore case when evaluating sign-ins. When `signInCaseSensitive` is false, Cognito will not
+check the capitalization of the alias when signing in. Default is true.
+
 ### Attributes
 
 Attributes represent the various properties of each user that's collected and stored in the user pool. Cognito

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

@@ -500,6 +500,13 @@ export interface UserPoolProps {
    * @default - No Lambda triggers.
    */
   readonly lambdaTriggers?: UserPoolTriggers;
+
+  /**
+   * Whether sign-in aliases should be evaluated with case sensitivity.
+   * For example, when this option is set to false, users will be able to sign in using either `MyUsername` or `myusername`.
+   * @default true
+   */
+  readonly signInCaseSensitive?: boolean;
 }
 
 /**
@@ -637,6 +644,9 @@ export class UserPool extends Resource implements IUserPool {
         from: props.emailSettings?.from,
         replyToEmailAddress: props.emailSettings?.replyTo,
       }),
+      usernameConfiguration: undefinedIfNoKeys({
+        caseSensitive: props.signInCaseSensitive,
+      }),
     });
 
     this.userPoolId = userPool.ref;

packages/@aws-cdk/aws-cognito/test/user-pool.test.ts

@@ -454,6 +454,36 @@ describe('User Pool', () => {
     });
   });
 
+  test('sign in case sensitive is correctly picked up', () => {
+    // GIVEN
+    const stack = new Stack();
+
+    // WHEN
+    new UserPool(stack, 'Pool', {
+      signInCaseSensitive: false,
+    });
+
+    // THEN
+    expect(stack).toHaveResourceLike('AWS::Cognito::UserPool', {
+      UsernameConfiguration: {
+        CaseSensitive: false,
+      },
+    });
+  });
+
+  test('sign in case sensitive is absent by default', () => {
+    // GIVEN
+    const stack = new Stack();
+
+    // WHEN
+    new UserPool(stack, 'Pool', {});
+
+    // THEN
+    expect(stack).toHaveResourceLike('AWS::Cognito::UserPool', {
+      UsernameConfiguration: ABSENT,
+    });
+  });
+
   test('required attributes', () => {
     // GIVEN
     const stack = new Stack();