client needs to send the `period-number` used in the blinding of its permanent identity

@yanmaani pointed out that in this section:

• https://gitlab.torproject.org/amjoseph/prop-onion-mutual-auth#new-extension-field-in-introduceencryptedencrypted_dataext_field

The server must verify that blinding the client-provided CLIENT_ONION_PUBLIC_IDENTITY_KEY produces the same signing key used in the client-provided descriptor-signing-key-cert. However the blinding process requires a period-number as input, and the client/server clocks may be skewed (or the period-number may increment while the message is in-flight).

It's quite ugly to force the server to test adjacent period-numbers (like TOTP validators do); it would be cleaner for the client to simply send the period-number and have the server verify that the received period-number is not implausibly far from its own clock. TODO: specify exactly what "implausibly" means.