Now we use GitHub Releases to detect when there's a new version of
codex-acp out, and we notify the user in the same way we do for the
other external agents.

This also moves `github_download.rs` out of the `languages` crate and
into `http_client`, because now we're not just using it for language
servers anymore, we're also using it for external agents.

Release Notes:

- N/A

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Co-authored-by: Lukas Wirth <me@lukaswirth.dev>