[Managed Application] Missing spec for a rest api method defined in hand-written docs.

In this [document](https://docs.microsoft.com/en-us/azure/azure-resource-manager/managed-applications/publish-managed-identity#accessing-the-managed-identity-token), we have a rest API method called listTokens and this method is not defined in [managed application's swagger](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/solutions/resource-manager/Microsoft.Solutions/stable/2021-07-01/managedapplications.json), we have customer asking SDK to provide the function to help get client id through this rest api method. Need service team attention and have this method included in swagger

Start with the linked managed identity documentation and specification/solutions/resource-manager/Microsoft.Solutions/stable/2021-07-01/managedapplications.json. Compare the documented listTokens method with nearby operations and confirm the service team’s expected request and response details. Done means the method is represented in the managed applications specification and passes the repository’s specification validation.

Written by the indexing model from the issue text.