Stream Duo Chat slash commands responses

What does this MR do and why?

Add streaming to responses from Duo Chat slash commands (i.e. /explain, /refactor, /tests). We put the streaming handler code in the shared concern AiDependant so we can use it everywhere is needed.

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

• Create a Personal Access Token on your local instance with at least api scope
• On VSCode with the GitLab extension (standalone app, Web IDE is not working for me): Remove all accounts, if any, from your extension. I don't know if this is needed, but if you only have credentials for your local instance you can be sure you're not reaching GitLab.com :)

• Add an account to the extension. Use http://127.0.0.1:3000 as your instance URL, and your access token as the credentials

• On the Gitlab Duo Chat tab, select some code and use one of the slash commands

Screen_Recording_2024-02-12_at_14.30.36

Closes #440310 (closed)