createTemplate creates the template definition; set isActive when it should
be immediately available to sendEmail.
Example
Inputs
templateName, subject, and registered sender are required. Provide exactly
one of inline htmlContent (at least 11 characters) or public htmlUrl.
Optional attachmentUrl, isActive, replyTo, tag, and personalized
toField are supported. account selects the Brevo binding.
Output
Returns{ id: number }. Creating a template does not send or test it and does
not consume a transactional send credit.