[REQ] [dart-dio-next] Enum fallback values

BuiltValue supports setting an enum entry as a fallback to use if the serialized enum value doesn’t exist.

This is great for backwards compatibility in APIs as an enum can fallback to an ‘unknown’ value.

## Describe the solution you'd like

In my API clients I generally modify the enum.mustache and inline_enum.mustache files to fallback to the first value.

@BuiltValueEnumConst({{#isInteger}}wireNumber: {{{value}}}{{/isInteger}}{{^isInteger}}wireName: r{{{value}}}{{/isInteger}}{{#-first}}, fallback: true{{/-first}})

A config flag could be added to enable this behaviour, or perhaps something more advanced like a config flag to determine which enum value to fallback to.

e.g. "enumFallbackValue": "unknown"

If an enum class is generated and has an "unknown" value, the fallback would be set on that value.

@kuhnroyal any thoughts?

[kuhnroyal]

@josh-burton This is fixed now 😃

[josh-burton]

@kuhnroyal yeah that would be great. Letting the user choose which value is the fallback would be the ideal solution.