Recover the user's forgotten user name.

Entitlements required: recoverUsername (Recover user name).

Verifies the user's identity by using the last step's method of verification and if successful, recovers the user's user name. When an OTP is not required, call this method with an empty body to complete the transaction and generate the audit event.

On Success, an email is sent to the user with the recovered username.
The email template for branding is at "authentication/login/cloud_directory/username/username_recovery.xml".
Pass in the themeId query parameter to brand the email template for notifications.