Consider splitting `message.rs` modules in `message.rs` and `models.rs`

[manuelmauro]

While messages are convenient structs encoding HTTP requests and responses models are working structs that are meant to be used from the end user of the SDK.

[ivnsch]

This task needs to be updated. Are we going to add separate domain objects everywhere, like we did for Transaction?

[manuelmauro]

It seems like a good solution to me, while I find the current organization ambiguous.