Trials

A trial gives a customer temporary, free access to a plan. It can be passed in during the billing call, or it can come with the plan configuration itself (which can be overriden).
Set a trial with free_trial on attach: { duration_length, duration_type (day|month|year), card_required, on_end }. Always pass duration_type.
Default to card not required (if there is no paid plan), and on_end: revert (if there is a paid plan).

If the customer the customer is NOT on a paid plan (free plan or no plan at all). 2 options:

No-card trial (default to this): attach with free_trial and set card_required false. The subscription starts with no card and ends at trial end if none is added. While on it, the customer cannot upgrade or attach another plan until they add a card via the Stripe billing portal.
Card-required trial: attach with free_trial and card_required: true and long_lived_checkout If the customer has no payment method, the attach returns a checkout URL to collect a card; they are charged when the trial ends. This should be done with a long-lived checkout URL param.

The customer already has an active (Stripe) subscription — common in sales-led trials.

On end: revert (default to this): attach the new plan with on_end: "revert" . This grants the plan in Autumn without touching the Stripe subscription; at trial end Autumn moves the customer back to their original plan, preserving the existing billing cycle.
On end: bill -- attaching a plan with a trial (or updating the subscription to add one) resets the Stripe billing anchor/cycle. This can be undesired so warn the user if they request this.
Card required param is ignored if there is already an active sub.