[dio] Test typed responses

[kuhnroyal]

This PR relates to #1335 and is for now just meant as idea and for further testing.
But correctly typed responses do work.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

[kuhnroyal]

@AlexV525 @ueman I had this open for a while and would like some feedback for 6.0.0 :)

[kuhnroyal]

Really would love some feedback here :)

[ueman]

I like it, good work

[AlexV525]

This is absolutely a significant step forward for the package. So everyone should write request<String?> instead of request<String>, right? If they upgraded without thinking, this might produce much NPE during the runtime?

[AlexV525]

The change is great, but we might evaluate the cost for the whole ecosystem.

[kuhnroyal]

I agree that this can break. So we need good examples in the 6.0.0 migration docs.

But users who have handling for nullable responses via response?.data or if (response.data != null) will receive compiler hints/warnings at these points and need to figure out what they want or expect. Good samples are the key here.

[kuhnroyal]

I added a changelog and some basic samples to the migration docs.
Really think we should do this. We have to do it at some point, might as well get it over with now :D

[AlexV525]

🚀