feat: add email thread support - Add threadId and inReplyTo fields to…

[pnyompen]

Add Email Thread Support

This PR adds support for email threading functionality to the Gmail MCP Server.

Changes

• Add threadId and inReplyTo fields to SendEmailSchema
• Add thread-related email headers (In-Reply-To, References)
• Enable thread ID specification when sending emails

Implementation Details

• Added proper type definitions for thread-related requests
• Updated email message creation to include thread headers
• Modified message sending logic to handle thread IDs

Testing

Please test the following scenarios:

1. Sending a new email in a thread
2. Creating a draft reply in a thread
3. Verifying thread headers in sent messages

Reference

Implementation based on Gmail API Thread Guide

[GongRzhe]

Great work! Thank you