Refactor all public structs (e.g. Payment) to interfaces with constructors

[alangalvino]

Description

It's impossible as of now to have any validation while constructing the public structs (e.g. Payment, Login, etc). For instance, if I wish to have a bool attribute that have the default value true I can't do it. That's because we are using the built-in new function to initialize some public structs.

We should create custom "constructors", e.g., NewPayment(...) as a way to have a better code encapsulation and maintainability. Here are a good reference of how to do it.

⚠️ This is a breaking change on the library.

Evidence

This PR has an example of the mentioned problem around bool default value.