Archive Conversation Feature in Flutter CometChat UIKit

Request to add an Archive Conversation feature in the CometChat UIKit. Currently, there is no built-in support for archiving chats.

Users should be able to archive conversations (primarily groups, but ideally both group and one-to-one chats) directly from the conversation list. Archived conversations should be moved out of the main list and accessible in a dedicated Archive section, with an option to unarchive them.