DnsExt: Resolve ManagerClass Type from IDnsProvider

[anpin]

Fixes #7726

Changes

DnsExt resolves ManagerClass Type from IDnsProvider instead of hardcoded SimpleDnsManager

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue #
• Changes in public API reviewed, if any.
• I have added website documentation for this feature.

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[anpin]

I couldn't find a specific test to run locally in order to verify if my change breaks anything, but that worked for me in another PR

[anpin]

seems to be a flaky unrelated test, is it not?

[Aaronontheweb]

seems to be a flaky unrelated test, is it not?

Yep, it's #7710 rearing its head again

[Aaronontheweb]

LGTM