feat(labrinth): environment variables for more customizable SMTP

[piprett]

This pull request adds two new environment variables, SMTP_PORT and SMTP_TLS. SMTP refers the protocol used to send emails.

SMTP_PORT configures which port to connect to on the SMTP server. It defaults to the standard and old default, 465.

SMTP_TLS is a boolean that defines if TLS security should be required to connect to the server. This defaults to the old value, true. Implementation wise I've copied the default values from the lettre crate we are using if the setting is enabled. This is how it behaved before, as we did not specify a TLS setting. If it is set to false it will instead use Tls::None.

This will help setting up labrinth for local development. You can use a mock SMTP server such as smtp4dev without setting up certificates and such.

[piprett]

Prioritizing this over #2883, making that one a draft and removing dependency on it due to conflicts.

[AlexTMjugador]

This PR addresses a real issue and is looking great overall, thank you! I'd like to take stewardship of it and work toward getting it merged soon. @Erb3, could you please update the branch and take a look at the comments below?

[piprett]

Good catches, I've updated the code. The default port is 465, and can be modified with SMTP_PORT. The SMTP_TLS now accepts the following values:

• none
• opportunistic_start_tls
• requires_start_tls
• tls

If none of these values are supplied to SMTP_TLS, it will default to TLS and throw a warning. I experimented with using an enum for the TLS setting, but in the end I used a simpler solution with a switch statement.

I have not tested the code with TLS as I don't have an email server. The none option has been tested with smtp4dev, and it works well with the following labrinth configuration:

SMTP_USERNAME=modrinth
SMTP_PASSWORD=frog
SMTP_HOST=localhost
SMTP_PORT=2525
SMTP_TLS=none

Thank you for your very friendly review!

[AlexTMjugador]

Splendid, thank you for your thoughtful PR and comments!

I've tested this on my end with Mailtrap’s STARTTLS-enabled SMTP development server, and everything worked great. It's also trivially clear that the direct TLS case has to function as expected, so we now have full coverage of all practical SMTP transport layer security protocols.

Would you be up for tweaking one last thing before merging?