Make all domain type fields public

[seanchen1991]

Proposition from @soareschen:

I'd like to propose that we make public all fields for all domain types in ibc-rs. Given that they all have auto-derived instances such as serde::{Serialize, Deserialize}, there are very little benefit of "protecting" the fields.

I think the original rationale for keeping the fields private is so that validation is always done via the constructors. This used to be the case for converting from raw protobuf to the domain types. But with the auto-derived Serialize instances added, it now leaves "backdoor" for constructing the domain types without validation.

It would only make sense to keep the fields private, if we also hand-derive all Serialize instances following the same validation rules. Otherwise, I think it only gives false sense of security that will eventually cause vulnerabilities to be introduced.

I also think that there are better ways to enforce validation at the type level aside from keeping fields private. For example, we could have a wrapper type like pub struct Validated(Value), where Value implements a trait like CanValidate, and that it is only possible to construct such wrapped value if the validation function passes. That way, you only use the Validated in places where validation is critical, while allowing unrestricted access for code that do not care as much about having valid data, such as for clients and tests.