feat(email-resend): add Custom headers for the Resend adapter

[fdurackbrooks]

After some trying (and failing) to enable unsubscribe headers, I found that the Resend adapter does not pass custom headers from the Payload sendEmail options to the Resend API. When attempting to send an email with custom headers, they are ignored because the mapPayloadEmailToResendEmail function creates a new object that excludes the headers property.

await payload.sendEmail({
  from: "Test <test@domain.com>",
  to: "jimmybillbob@example.com",
  subject: "Resend adapter does not allow custom headers :(",
  html: html,
  headers: {
    "List-Unsubscribe": "<https://domain.com/unsubscribe>",
    "List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
  },
});

I have recitified this by adding headers: message.headers, to the mapPayloadEmailToResendEmail in packages/email-resend/src/index.ts, as below.

function mapPayloadEmailToResendEmail(
  message: SendEmailOptions,
  defaultFromAddress: string,
  defaultFromName: string,
): ResendSendEmailOptions {
  return {
    // Required
    from: mapFromAddress(message.from, defaultFromName, defaultFromAddress),
    subject: message.subject ?? '',
    to: mapAddresses(message.to),

    // Other To fields
    bcc: mapAddresses(message.bcc),
    cc: mapAddresses(message.cc),
    reply_to: mapAddresses(message.replyTo),

    // Optional
    attachments: mapAttachments(message.attachments),
    html: message.html?.toString() || '',
    text: message.text?.toString() || '',
    headers: message.headers, // Added this line.
  } as ResendSendEmailOptions
}