Implement the Notification Repository

[raaaahman]

The Repository will allow to query, store or update the notifications status: https://github.com/WordPress/wp-notify/blob/develop/includes/persistence/class-wp-notify-wpdb-notification-repository.php

It should be configured with a connection to an actual persistence mechanism (like a MariaDB database) but be decoupled from it.

A tutorial about how to implement a Repository using WordPress queries: https://carlalexander.ca/designing-class-manage-wordpress-posts/

• The repository should be able to query notifications by a date range (e.g.: last month, last week, etc.)
• The queried notifications should be filtered to only return notifications with a recipient matching the current user.