[dart][dart-dio] Add type information to oneOf and anyOf

[fa0311]

We will stop using OneOfDynamic and switch to OneOfx.
This will improve type safety.

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

[fa0311]

Unfortunately, the dart-dio generator seems to lack sufficient tests, so it's unclear whether it works correctly in all cases.
In my case, it works fine.

[ahmednfwela]

can't this case just return OneOfDynamic to be backwards compatible?

[ahmednfwela]

meant here

[fa0311]

We can maintain backward compatibility, but a simple replacement won't work.
If we simply return OneOfDynamic, it will fail when casting to OneOfx (OneOf{{oneOf.size}}), resulting in an error.
A slightly more involved approach would be to introduce a flag for cases with seven or more oneof values—if set to true, it would use OneOfDynamic.

[ahmednfwela]

To be honest, I never really liked the idea of fitting infinitely large oneof/anyof cases into a single variable

The approach I followed here #18970 is much cleaner, and is similar to what protobuf does