ci(build-channel): create types to represent data used in build-channel script

> Rather than writing directly to the toml document, one alternative approach that might be a bit more type-safe is to create a Rust type to represent the data you want, populate that, then serialize that type to `toml` afterwards. This helps to avoid typos in strings, and you get the side-benefit of being able to use the same type for deserialization.

_Originally posted by @mitchmindtree in https://github.com/FuelLabs/fuelup/pull/337#discussion_r1071718044_

This would involve creating structs for some of the usages of `toml_edit::value` ([example](https://github.com/FuelLabs/fuelup/blob/8ca26c1378112d79df733d62a4caef85816c3970/ci/build-channel/src/main.rs#L146)), eg. a `Channel`, then serializing the `Channel`.