Allow optional: true to then skip the requires_tenant setting

[gommo]

We ran into a use case that I'd like to see if others would find useful.
We have patched active_storage to be tenanted however there are other gems like action_mailbox which use storage (and how we route we don't have the tenant initially)
I was thinking it may be useful to honor the optional: true on the acts_as_tenant option to then skip requiring a tenant in that case.

[nightcoding]

This would be very helpful! I currently have this requirement:

when a new user signs up, a tenant is not yet needed. If sign up was successful, the user has to create the tenant. After some confirmations (manually, because legally required to) the require_tenant is neccessary on every user update.

It would be very helpful if I could disable the validation on new records like with ActsAsTenants.without_tenant do ... end.