feat: thread google chat messages (fixes #27823)

What does this MR do and why?

Threads messages sent to the Google Chat integration with the proper context. For example, notes and issue changes on a given issue will appear as a single chat thread rather than disparate messages.

Screenshots or screen recordings

How to set up and validate locally

1. Enable Google Chat (hangouts) integration for a project
2. Create a new issue within the project
3. Add some comments to the issue, close it, reopen, etc
4. Similarly for merge requests
5. Push multiple changes to the default branch

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

app/models/integrations/hangouts_chat.rb

@@ -47,8 +47,28 @@ def default_fields
     private
 
     def notify(message, opts)
+      url = webhook
+      key = parse_thread_key(message)
+      if key
+        url += "&threadKey=" + CGI.escape(key)
+      end
+
       simple_text = parse_simple_text_message(message)
-      ::HangoutsChat::Sender.new(webhook).simple(simple_text)
+      ::HangoutsChat::Sender.new(url).simple(simple_text)
+    end
+
+    # Returns an appropriate key for threading messages in google chat
+    def parse_thread_key(message)
+      case message
+      when Integrations::ChatMessage::NoteMessage
+        message.target
+      when Integrations::ChatMessage::IssueMessage
+        "issue #{Issue.reference_prefix}#{message.issue_iid}"
+      when Integrations::ChatMessage::MergeMessage
+        "merge request #{MergeRequest.reference_prefix}#{message.merge_request_iid}"
+      when Integrations::ChatMessage::PushMessage
+        "push #{message.ref}"
+      end
     end
 
     def parse_simple_text_message(message)