Validates the authentication attempt that is associated with the current step in the username recovery flow.

Entitlements required: recoverUsername (Recover user name).

The validator must be called for each step, until only one step remains.
After a successful authentication, the step is marked complete and the API initiates the next step. The response indicates the number of steps that remain and the next step to continue. When only one step remains, call the PUT //v1.0/usc/username/recovery/{trxId} API to recover the user's user name and complete the transaction.