Endpoints
GET /campaigns/:idPATCH /campaigns/:id
from.address requires a verified domain in the current project; assigning audiences and a template version also requires them to belong to that project.
Read campaign readiness and update an email campaign draft.
GET /campaigns/:idPATCH /campaigns/:idfrom.address requires a verified domain in the current project; assigning audiences and a template version also requires them to belong to that project.