[notifications] Email template and dispatch notification integration

[davidgamez]

Description

Create the email templates and connect each notification type to the notification dispatcher. The template need to support digest mode only for MVP. In digest multiple notifications from the same type are included in the message. (Not MVP) in no-digest mode a single notification per notification type is sent.

The dispatcher should select the appropriate template, populate it with event data, and send notifications through Brevo based on user subscriptions.