Follow / Unfollow thread support in EmbeddedChat

[Pratheek555]

Brief Title

Enable users to follow and unfollow thread messages to receive updates

Acceptance Criteria fulfillment

• [ ] Added UI control to follow and unfollow a thread
• [ ] Users can follow a thread without posting a message
• [ ] Shows exactly two avatars amongst the list of users, and rest are shown in a "+2" users format.

Fixes #1062

Video/Screenshots

https://github.com/user-attachments/assets/9575582c-7912-47cf-9f7b-28afa7a474d4

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-<pr_number> after approval. Contributors are requested to replace <pr_number> with the actual PR number.