[BUG][dart-dio] Avoid depending on Equatable implementation files

[LukasMirbt]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

With equalityCheckMethod: equatable, an implementation file (package:equatable/src/equatable_utils.dart) is imported and the internal equals and mapPropsToHashCode functions are used.

equals was recently removed in Equatable v2.0.6 and then reverted in v2.0.7.

Suggest a fix

Would it be possible to use Equatable through the public interface instead of depending on internal code that may break again in the future?