collab: Add usage-based billing for LLM interactions (#19081)

This PR adds usage-based billing for LLM interactions in the Assistant.

Release Notes:

- N/A

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Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>