Support HTTP API email providers (SendGrid, Resend, ...) for self-hosting

[ksk-ktsr]

Problem

Self-hosted Nango can only send transactional email (verification, invites, usage notifications) through Mailgun's API or SMTP:

// packages/email/lib/client.ts
if (envs.MAILGUN_API_KEY) {
    this.provider = new MailgunEmailProvider();
} else if (envs.SMTP_URL) {
    this.provider = new SmtpEmailProvider();
} else {
    this.provider = new NoEmailProvider();
}

In deployments where outbound SMTP is blocked and Mailgun isn't the vendor of choice, there is no working configuration. NoEmailProvider only logs, so those emails are silently never delivered.

Proposal

Add a generic HTTP provider implementing the existing EmailProvider<T> interface, configured entirely by env vars, so any JSON-over-HTTP mail API (SendGrid, Resend, Postmark, ...) can be used without a vendor-specific module. No new dependency is needed — fetch is built in on the supported Node (>= 20), unlike the Mailgun path which pulls mailgun.js + form-data.

Rough shape:

EMAIL_HTTP_URL=https://api.sendgrid.com/v3/mail/send
EMAIL_HTTP_HEADERS={"Authorization":"Bearer <key>"}
EMAIL_HTTP_BODY=<JSON template with {{to}} / {{from}} / {{subject}} / {{html}}>

Selection would slot into the existing precedence chain after SMTP, so behaviour is unchanged for anyone already configured.

Questions before I open a PR

1. Is a generic template-driven provider the direction you'd want, or would you rather see a vendor-specific SendGridEmailProvider sitting next to MailgunEmailProvider?
2. packages/email has no tests today (vitest is already a devDependency). I'd add the first ones covering this provider — any objection?

Happy to send the PR once you confirm the direction.

[linear-code]

NAN-6490