PendingUsernames: StorageDescriptor<
    [Key: Binary],
    [SS58String, number],
    true,
    never,
>

Usernames that an authority has granted, but that the account controller has not confirmed that they want it. Used primarily in cases where the AccountId cannot provide a signature because they are a pure proxy, multisig, etc. In order to confirm it, they should call [Call::accept_username].

First tuple item is the account and second is the acceptance deadline.