fix: various issues with API structs

[csatib02]

This PR addresses issue #26. The problem described in the ticket occurs because the omitempty directive in the JSON struct tag instructs the encoding/json library to omit fields with a "zero value." The zero value depends on the field's type; for structs, the zero value is an initialized but empty struct.

Solution:

• To resolve this issue, all struct fields have been changed to pointers. This ensures that the omitempty directive omits fields that are nil, as the zero value for a pointer is nil.
• There were a couple of cases where it was possible to cause a nil-pointer panic.
• Added several test cases to verify that the implementation works as intended.

NOTE: The reason why yaml marshalling worked, is becuase yaml has a different implementation for the omitempty tag. It treats empty collections (like maps and slices) as well as empty but initialized structs as "omittable," even when non-nil..

[pepov]

Nice one!

[OverOrion]

Nice, thank you very much! 🚀