feat(notif): email templating

[seanaye]

This PR adds the capability to format emails for the digest, given an input DigestBatch we can format all of the notifications into an email for the user.

We then integrate this formatting with the notification sandbox

• add email formating crate
• implement the email templating
• update the template
• wrap over email_formatter email in sandbox

[whutchinson98]

This crate seems pretty specific to notification emails. Should probably be named notification_email_formatting instead imo

[seanaye]

This crate seems pretty specific to notification emails. Should probably be named notification_email_formatting instead imo

Ill change in a follow up to avoid confilcts