[dart-dio] Adds support for generating inline enums

[josh-burton]

Adds support for enums declared inline. Previously these would just generate as a string field. Now the enum is generated within the model class.

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CC: @ircecho @swipesight @jaumard @nickmeinhold @athornz @amondnet

[jaumard]

Why handle only inline enum and not reusable enums ?

[josh-burton]

Why handle only inline enum and not reusable enums ?

Reusable enums (enums declared as a definition are already handled. The model.mustache template redirects to either class.mustache or enum.mustache depending on whether the definition is an enum or not.

[wing328]

If no further feedback on this PR, I'll merge it tomorrow (Thur)

[kuhnroyal]

@josh-burton There are missing imports for built_value collection due to the used BuiltSet in the inline enum in some cases.