[dart][dart-dio] Improved library specific template usage

Improved library specific template usage. This is just an idea on how to make templating for different libraries a little bit easier. With this the main templates don't have to be changed when adding a new library. It could be used for the new json_serializable library option that is happening in some branch.

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

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