Put mail handler component into a separate crate

[wojcik91]

We have the initial setup for migrating to a multi-crate workspace, but we need to finally start pulling out parts of defguard_core.
Mail handler seems like a good candidate since it's already largely independent and works by processing messages from a channel.

Goal:

• put mail handler into a separate defguard_mail crate
• trigger sending emails through the event router