Feat(smtp): Add SMTP integration to 12 Factor

[alithethird]

This PR is related to 12 Factor SMTP PR.

Rationale

Some applications require an SMTP integration to send emails to their clients. This PR is a follow-up to the latest 12 Factor PR for adding SMTP integration.

Changes

• Add commented-out SMTP relation required objects to extension charmcraft.yaml.j2 files.
• Add references about SMTP into extension reference documents.
• Add documentation about SMTP integration and generated environment variables
• Add the smtp charm lib to the initializer.

Reviewers:
@jdkandersson
@javierdelapuente
@erinecon

[erinecon]

Thanks for making this part of the reference reusable! Some small comments from me :D

[medubelko]

Leaving the TA review to @jahn-junior and @erinecon.