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>

app/javascript/dashboard/api/inbox/conversation.js

@@ -68,6 +68,10 @@ class ConversationApi extends ApiClient {
     return axios.post(`${this.url}/${id}/update_last_seen`);
   }
 
+  markMessagesUnread({ id }) {
+    return axios.post(`${this.url}/${id}/unread`);
+  }
+
   toggleTyping({ conversationId, status, isPrivate }) {
     return axios.post(`${this.url}/${conversationId}/toggle_typing_status`, {
       typing_status: status,