Add a stable re-export surface and tests for external rack-init tooling

[sion42x]

External testing tools have either hand-maintained config-rss.toml or duplicated the request types, and both break silently when the format moves. This (tiny) crate re-exports everything a RackInitializeRequest is built from and renders the config-rss.toml sled-agent parses.

The tests round-trip constructed requests and the in-tree non-gimlet example through sled-agent's rack_initialize_request_from_file, giving us a CI-side test to catch breaking changes in different scenarios:

1. Wire-only rename via #[serde(rename)] won't fail on CI or in external tools
2. Rust-visible rename will fail on CI, the dev will need to rename the field in the literal and nothing happens in external tools until we bump the rev
3. Shape change, like string -> struct or Vec -> named pool objects (prior breaking issues). In omicron there will be a type mismatch in the literal, so a new shape needs to be constructed, and we will need to update external tools as well to handle the new non-derivable data.

[andrewjstone]

Looks great!

[andrewjstone]

This should use the "2FAKE00{index}" format for consistency with everything else.