zeta cli: Add convert-example command (#41608)

Adds a `convert-example` subcommand to the zeta cli that converts eval
examples from/to `json`, `toml`, and `md` formats.

Release Notes:

- N/A

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

Cargo.lock

@@ -21757,6 +21757,7 @@ dependencies = [
  "polars",
  "project",
  "prompt_store",
+ "pulldown-cmark 0.12.2",
  "release_channel",
  "reqwest_client",
  "serde",
@@ -21766,6 +21767,7 @@ dependencies = [
  "smol",
  "soa-rs",
  "terminal_view",
+ "toml 0.8.23",
  "util",
  "watch",
  "zeta",