make deserializable fields pub

[sunshowers]

Currently we have a number of structs that are:

• deserializable
• don't have any kind of validation as part of deserialization
• have private fields

Generally speaking, private fields like this are misleading, since the fields are defacto public through a constructed serde_json::Value or similar. See for example oxidecomputer/omicron#9723.

We should audit our uses and make the fields either public or accessible via a constructor.