add salt to FrontendCognitoConfig to make it always run (#1674)

### Feature or Bugfix
- Bugfix

### Detail
Problem:
We modify the Cognito UserPoolClient manually with CustomResources (not
through CDK deployment) and as such everytime we make a change to the
UserPoolClient CDK code (i.e enabling/disabling integtests, changing the
timeout etc) the new deployment is overrriding the manual changes.
We could specify the callback URLs in CDK but we don't know the URLs
until later in the pipeline when the CDN is being deployed. Additionally
AuthAtEdge requires Cognito to be already setup and as such we have a
chicken and egg situation.

Solution:
Adding a random salt (timestamp) as an environment variable to force
triggering the CustomResource on every deployment.
The downside of this solution is that the particular stack will always
create a diff but due to the structure of the stacks it is very
complicated to work around the issue.

### Security
Please answer the questions below briefly where applicable, or write
`N/A`. Based on
[OWASP 10](https://owasp.org/Top10/en/).

- Does this PR introduce or modify any input fields or queries - this
includes
fetching data from storage outside the application (e.g. a database, an
S3 bucket)?
  - Is the input sanitized?
- What precautions are you taking before deserializing the data you
consume?
  - Is injection prevented by parametrizing queries?
  - Have you ensured no `eval` or similar functions are used?
- Does this PR introduce any functionality or component that requires
authorization?
- How have you ensured it respects the existing AuthN/AuthZ mechanisms?
  - Are you logging failed auth attempts?
- Are you using or adding any cryptographic features?
  - Do you use a standard proven implementations?
  - Are the used keys controlled by the customer? Where are they stored?
- Are you introducing any new policies/roles/users?
  - Have you used the least-privilege principle? How?


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

deploy/stacks/frontend_cognito_config.py

@@ -1,3 +1,5 @@
+from datetime import datetime
+
 from aws_cdk import (
     aws_lambda as _lambda,
     aws_iam as iam,
@@ -105,6 +107,7 @@ def __init__(
                 'envname': envname,
                 'deployment_region': backend_region,
                 'custom_domain': str(bool(custom_domain)),
+                'timestamp': datetime.utcnow().isoformat(),
             },
             environment_encryption=lambda_env_key,
             tracing=_lambda.Tracing.ACTIVE,