DEV: chat streaming (#25736)

This commit introduces the possibility to stream messages. To allow plugins to use streaming this commit also ships a `ChatSDK` library to allow to interact with few parts of discourse chat.

```ruby
ChatSDK::Message.create_with_stream(raw: "test") do |helper|
  5.times do |i|
    is_streaming = helper.stream(raw: "more #{i}")
    next if !is_streaming
    sleep 2
  end
end
```

This commit also introduces all the frontend parts:
- messages can now be marked as streaming
- when streaming their content will be updated when a new content is appended
- a special UI will be showing (a blinking indicator)
- a cancel button allows the user to stop the streaming, when cancelled `helper.stream(...)` will return `false`, and the plugin can decide exit early

config/initializers/000-zeitwerk.rb

@@ -44,6 +44,7 @@ Rails.autoloaders.each do |autoloader|
     "ssrf_detector" => "SSRFDetector",
     "http" => "HTTP",
     "gc_stat_instrumenter" => "GCStatInstrumenter",
+    "chat_sdk" => "ChatSDK",
   )
 end
 Rails.autoloaders.main.ignore(