FEATURE: allows to force a thread (#25987)

Forcing a thread will work even in channel which don't have `threading_enabled` or in direct message channels.

For now this feature is only available through the `ChatSDK`:

```ruby
ChatSDK::Message.create(in_reply_to_id: 1, guardian: guardian, raw: "foo bar baz", channel_id: 2, force_thread: true)
```

plugins/chat/app/controllers/chat/api/channel_messages_controller.rb

@@ -31,6 +31,9 @@ class Chat::Api::ChannelMessagesController < Chat::ApiController
   end
 
   def create
+    # users can't force a thread through JSON API
+    params[:force_thread] = false
+
     Chat::MessageRateLimiter.run!(current_user)
 
     with_service(Chat::CreateMessage) do