feat: Allow users to mark a conversation as unread (#5924)

Allow users to mark conversations as unread.
Loom video: https://www.loom.com/share/ab70552d3c9c48b685da7dfa64be8bb3

fixes: #5552

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

config/routes.rb

@@ -85,6 +85,7 @@ Rails.application.routes.draw do
               post :toggle_status
               post :toggle_typing_status
               post :update_last_seen
+              post :unread
               post :custom_attributes
             end
           end