[Feature] Email Subscription system

[rajatkb]

Create a new Controller and endpoint to trigger a mail to the respective, Email provided through the rest end point. So a user would be able to hit the rest end point /subscribe/email?em=xyz@yl.com with a payload of filters and will get updates of whenever new conferences are entered into the system or updates to existing one's are done.

Describe the solution you'd like
One hitting the rest endpoint the controller would register a new email along with it's respective filters. There will be a new listener attached to changeStream events and the respective event data will be analyzed based on filters for emails. Then a notification for these mails will be sent.

Additional context

REQUIREMENTS

• Create a Proper extensible Mongo Schema for representing an email. (Mongo Model)
• Create proper filter parameters for the payload of registering an email. ( Controller / Service)
• Create a new Listener for filtering out emails for individual event of new Conference data insertion/updates. Leverage mongodb query to get the emails filtered. (Listener)