[Feat]: Add a "Mentions" tab where users can see all the messages in which they were mentioned.

[Spiral-Memory]

As an EmbeddedChat developer,

I need to:
Implement another sidebar tab "Mentions" where users can see all the messages in which they have been mentioned.

So That:
Users will be able to easily find out the messages in which they were mentioned in a busy channel with lots of messages.

Acceptance Criteria:

• Add a new menu named "Mentions."
• Look for an appropriate REST API endpoint to fetch all mentions.
• Implementing the same in the codebase.

[Spiral-Memory]

@abhinavkrin , I am working on this feature. In the meantime, please let me know your thoughts on whether this feature is needed in the embedded chat app and whether it would be useful for the users.