[dart-dio] Fix json_serializable response for type:string format:binary

[matthewnitschke-wk]

Summary

Using the dart-dio generator against a response of type:string, format:binary, would result in a MultipartFile return type. This causes the deserialize function to fail.

This PR takes inspiration from the built_value serialization method by treating response file types as Uint8List

Closes #20682

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  …ry response types

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[matthewnitschke-wk]

In a previous iteration I implemented the same logic as the built_value serialization method, where both parameter and return types are considered Uint8List

In an effort to keep this change slightly less impactful, I've opted to just change the return type when the response is a file

I'm open to input on this change, whether we want parity with built_value (both parameters and return types are Uint8List) or are ok with what I have here

[matthewnitschke-wk]

@wing328 Ended up creating a specific test case for this to prevent effecting non-dart-dio generators: bin/configs/dart-dio-binary-response.yaml

Can you approve the workflow run so we can see if tests pass?