feat(fetch): support X-GM-THRID, X-GM-MSGID, X-GM-LABELS in FetchParser

[dev10m-adv]

📌 Description

This update enhances the IMAP fetch parsing by adding support for Gmail-specific extensions:

X-GM-THRID → Gmail thread ID
X-GM-MSGID → Gmail unique message ID
X-GM-LABELS → Gmail labels associated with the message

These fields are now parsed directly within the FetchParser, making them available alongside standard IMAP attributes.

🚀 Motivation

Gmail provides additional metadata that is not part of the standard IMAP specification but is essential for building a modern email client:

Thread grouping (conversation view)
Reliable message identification across folders
Label-based organization (instead of traditional folders)

Previously, this data was not accessible through the parser, limiting Gmail-specific functionality.

🔧 Changes
Extended FetchParser to recognize and extract:
X-GM-THRID
X-GM-MSGID
X-GM-LABELS
Properly map parsed values into the fetch result structure
Ensure backward compatibility with non-Gmail IMAP servers
✅ Impact
Enables proper threading support (conversation view)
Improves message deduplication and syncing
Allows accurate label synchronization for Gmail accounts
No breaking changes for existing IMAP functionality
⚠️ Notes
These fields are Gmail-specific and may not be present in other IMAP providers
Consumers should handle null/absence cases gracefully
🧪 Testing
Verified against Gmail IMAP (imap.gmail.com)
Tested parsing of:
Messages with multiple labels
Threaded conversations
Standard IMAP responses (non-Gmail) to ensure no regression

[robert-virkus]

Thanks for your contribution and sorry for the long response time!