[dart-dio] Adds support for enumUnknownDefaultCase to BuiltValue enums

[josh-burton]

Adds support to dart-dio and built value for the new enumUnknownDefaultCase flag. When this flag is enabled, a default enum value is created.

With this change, that value is now set as the fallback value when deserializing unknown enums. This prevents runtime errors when the deserializer encounters an unknown enum.

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