title: Fine Grained access control for dynamodb does not work for LeadingKeys:${aws:username}
body: <p>I'm just trying out a POC for my use-case so I'm still trying to understand a lot of new concepts.</p>
  <p>I created a user with username adam123 and the dynamo db table has a partition key and a sort key of say username and some address. So one of the items include:</p>
  <p>username: adam123
  address: 123 street</p>
  <p>I gave the user limited dynamodb access using fine grained control (used LeadingKeys condition) and specified LeadingKeys:${aws:username}. But it still says access denied.</p>
  <p>I want to understand if I want a dynamic use-case for LeadingKeys, is there a limitation on using on userID, cognito credentials? Can I use say session tags or username</p>