agent: Add sound notification when done generating (#31472)

This PR adds the ability to hear a sound notification when the agent is
done generating and/or needs user input. This setting is turned off by
default and can be used together with the visual notification. The
specific sound I'm using here comes from the [Material Design 2 Sound
Library](https://m2.material.io/design/sound/sound-resources.html#).

Release Notes:

- agent: Added the ability to have a sound notification when the agent
is done generating and/or needs user input.

Cargo.lock

@@ -60,6 +60,7 @@ dependencies = [
  "assistant_slash_commands",
  "assistant_tool",
  "async-watch",
+ "audio",
  "buffer_diff",
  "chrono",
  "client",