[Provider] Add AutoSend as a built-in email provider

[sauropeerlist]

Goals

1. Let developers using AutoSend send magic link sign in emails without writing a custom sendVerificationRequest
2. Provide first-class Auth.js integration for AutoSend, the same as the other email providers
3. Auto-detect the API key from AUTH_AUTOSEND_KEY so a basic setup needs almost no config

Non-Goals

1. No SDK dependency, just a single fetch so @auth/core stays dependency-free
2. Not changing the shared email flow (token creation, adapter storage, callback)

Background

Auth.js supports email based sign in out of the box and already ships several HTTP email providers (Resend, Sendgrid, Postmark, Loops, Mailgun). Anyone using an email vendor that is not on that list has to wire up a custom provider by hand and reimplement the same flow, which is repetitive and easy to get wrong. AutoSend is a transactional and marketing email platform that works the same way as those existing providers, so it fits the pattern rather than needing anything new. There is no built-in AutoSend option today short of writing your own provider. Docs at docs.autosend.com.

Proposal

Add AutoSend as a named, built-in provider in @auth/core, following the same pattern as the other email providers. I've already implemented and tested it locally (request shape, full sign in flow with an adapter, and a real send), so I'm happy to contribute the PR.

[sauropeerlist]

Fix in PR: #13481