Add a new trigger to a workflow. The event_type_key must be a valid workflow
trigger event key — an invalid value returns 422. All condition FK fields accept
the integer id or obfuscated public_id of a referenced resource in the same
workspace/team scope.

Condition fields go directly on workflows_trigger
({"workflows_trigger": {"event_type_key": "$optin", "page_id": 123}}). A
trigger field nested in another object, sent beside workflows_trigger, or
given an object or list as its value returns 422 naming the field, and no
trigger is created. A condition left out matches every value, so the response
echoes each condition FK (null when unscoped).