fix(firestore-send-email) restore headers support

[Nushio]

I just noticed that this feature was removed, and wanted to contribute a quick fix.

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[Nushio]

I've signed the CLA

[Copilot]

Pull Request Overview

This PR restores support for custom email headers in the Firestore email sending extension by reintroducing the missing headers field and updating documentation accordingly.

• Restores the headers property in the mail options in index.ts.
• Bumps the extension version to 0.2.4 in extension.yaml.
• Updates documentation and changelog to reflect this new feature.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
firestore-send-email/functions/src/index.ts	Adds the headers field to email payload to support custom headers.
firestore-send-email/extension.yaml	Updates the extension version.
firestore-send-email/README.md	Adds a section on using custom headers with example JSON.
firestore-send-email/CHANGELOG.md	Documents the addition of the headers field and updates the version.

[cabljac]

Hi, can you run npm run generate-readme in the firestore-send-email/functions directory

[Nushio]

I ran it but it removes the blurb I wrote about using Custom Headers.

Where should I place that instead? PREINSTALL.md ?

Edit: Done