[Feature Request] Sendinblue Transactional API

Hi, can you please add support for Sendinblue Transactional API. Sendinblue has higher free quota compared to the rest (Sendgrid, mailgun, etc). Free plan offers 300 daily emails so it is good to have this added into Post SMTP plugin. Previously I used this plugin for Sendgrid API but sadly had to delete it and change to other smtp plugin that support Sendinblue API.

Really love post smtp for it features. Please consider this request. thank you.
API Doc
https://developers.sendinblue.com/docs/send-a-transactional-email