Feature: Implement CRUD Endpoint for Mails in Letter Service

[f-s-h]

Implement the following endpoint:

Method	Endpoint	Success Code	Error Codes	Description
POST	/mail	201 Created	400	Send a new email.

• Protocol: JSON over HTTP.
• Headers: All requests sending data and all responses must include Content-Type: application/json.

Sending an email first should send an email to all members.