chore(ec2): add VPC interface endpoints for Cognito (#35984)

Add new VPC inteface endpoints.

Ref: [Amazon Cognito user pools now supports private connectivity with AWS PrivateLink](https://aws.amazon.com/about-aws/whats-new/2025/11/amazon-cognito-user-pools-private-connectivity-aws-privatelink/)


```bash
$ aws ec2 describe-vpc-endpoint-services --filters "Name=service-name,Values=*cognito*" --region us-east-1 --query "ServiceNames[]"

[
    "com.amazonaws.us-east-1.cognito-idp",
    "com.amazonaws.us-east-1.cognito-idp-fips"
]
```

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: mazyu36

packages/aws-cdk-lib/aws-ec2/lib/vpc-endpoint.ts

@@ -462,6 +462,8 @@ export class InterfaceVpcEndpointAwsService implements IInterfaceVpcEndpointServ
   public static readonly CODEPIPELINE = new InterfaceVpcEndpointAwsService('codepipeline');
   public static readonly CODESTAR_CONNECTIONS = new InterfaceVpcEndpointAwsService('codestar-connections.api');
   public static readonly CODE_CONNECTIONS = new InterfaceVpcEndpointAwsService('codeconnections.api');
+  public static readonly COGNITO_IDP = new InterfaceVpcEndpointAwsService('cognito-idp');
+  public static readonly COGNITO_IDP_FIPS = new InterfaceVpcEndpointAwsService('cognito-idp-fips');
   public static readonly COMPREHEND = new InterfaceVpcEndpointAwsService('comprehend');
   public static readonly COMPREHEND_MEDICAL = new InterfaceVpcEndpointAwsService('comprehendmedical');
   public static readonly COMPUTE_OPTIMIZER = new InterfaceVpcEndpointAwsService('compute-optimizer');