Determine where semantics of fields will be validated

[andresuribe87]

Structs like what was added in #37 allow consumers of this SDK to parse any JSON message or resource that's part of tbdex. Some fields in a message are strings that are meant to represent numbers. As stated in #37 (comment) there seems to be a need to validate that the values of certain fields. For example, and "amount" fields should likely only have possible amount values, even thought it's serialized as a string in JSON.

Importantly, @amika-sq mentions that

One thing I do want to mention though is that tbDEX messages/resources will need to traverse the language boundary from Rust -> other foreign languages. Strings work really well in this regard, third-party crate types not so much.

It's definitely going to be some tradeoffs, but I think we can find a happy medium here. Looking forward to discussing this more!

@amika-sq , would you be able to elaborate on what the constraints are when passing around structs between language boundaries?