Use `@pragma('vm:awaiter-link')` for better stacktraces

[ueman]

Whenever we use a Completer which we do rather often, we can annotate it with @pragma('vm:awaiter-link') to get better stacktraces. We should do so. An in-detail explanation for the pragma can be found here.

[AlexV525]

@ueman https://github.com/cfug/dio/pull/2292/files#r1932184629

[AlexV525]

#2292 (comment)