Allow running zeta evals against sweep (#43039)

This PR restructures the subcommands in `zeta-cli`, so that the
prediction engine (currently `zeta1` vs `zeta2`) is no longer the
highest order subcommand. Instead, there is just one layer of
subcommands: `eval`, `predict`, `context`, etc. Within these commands,
there are flags for using `zeta1`, `zeta2`, and now `sweep`.

Release Notes:

- N/A

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Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Agus <agus@zed.dev>

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