feat(DuoChat): Improve the message response while generating

What does this MR do?

It's been noted that while Duo is still streaming a response in a message that it can be unclear if it is still working. This change uses a loading icon to subtly communicate that Duo is still generating a response. It also waits to give the option to add feedback until it is finished writing the message.

Closes gitlab#434437 (closed)

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