add an reactive stream as default fetcher?

[todo]

https://github.com/Mikibot/Miki.Framework/blob/dca9eb17e71143f9ba221e62ff897d35f6fb5700/src/Miki.Framework/IProvider.cs#L10-L15

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This issue was generated by todo based on a TODO comment in dca9eb1. It's been assigned to @velddev because they committed the code.