Add an api to re-send verification mail

[CliffHan]

Description

For now, the verification email may only be sent after a successful user registration.

There should be an api for the user to re-send verification token.

To Reproduce

refer to: https://github.com/loco-rs/loco/blob/master/loco-new/base_template/src/controllers/auth.rs#L65C17-L65C29

No other function called AuthMailer::send_welcome() except register()

Expected Behavior

In the controller, add an endpoint, maybe /auth/resend-verification-mail, or whatever.

It will make the /auth part more complete.

[CliffHan]

And maybe the "Authentication Configuration" could be extended with something like a allow_only_verified_email_to_login.