Add an alternative to a proposal

Adds an alternative to a proposal, built around one package, and returns the new alternative. Alternatives are the options a client chooses between: each one holds its own packages and price, and the client books one of them.

The first call on a proposal creates two alternatives, not one. The packages already on the proposal move into an alternative of their own, and the new package goes into a second. After that, each call adds one more.

To put more packages in an alternative, add them with POST /api/v1/events/{event_id}/event_bookings and pass its proposal_alternative_id. To start from a copy of an existing alternative, use POST /api/v1/events/{event_id}/proposal_alternatives/{id}/duplicate instead.

Adding an alternative resets any custom payment plan on the proposal.

Only event_type_id and starts_at_date are always needed. Whether the rest apply depends on how the package is sold, which GET /api/v1/packages/{id} tells you.

Required permission: assigned_event_write (when the authenticated user is assigned to the event) or unassigned_event_write (otherwise), together with the matching read permission for the event itself.