title: AWS Cognito custom static claims for all users in user pool
body: <h2>Context:</h2>
  <p>I am integrating AWS Cognito into an existing multi-tenant application with preconfigured tenant ids. I am currently trying to find a way to add a custom static claim to each user in a given user pool. The claim is the <code>tenant_id</code>. This claim would have the same immutable value for each user in a user pool.</p>
  <h2><strong>Question:</strong></h2>
  <p>Is there a way to add a custom static claim like this for each user in a user pool?</p>
  <h2><strong>Reason:</strong></h2>
  <p>I would like to provide a stateless way to interact with existing services that currently only know of the existing tenant_id.</p>