Add async methods and examples

[circler3]

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I add async methods and examples for those who want to use it in async scenarios.

[Vect0rZ]

@circler3 Thank you for your contribution. The change is generally safe to merge, but I will need some time to give it a test and review it locally. If everything is alright, will merge it. One thing I noticed is that the only frame processing method that is async is OnStreamFrameAsync and we might want to update the other ones. This might pollute the QuicConnection class, but I think it's okay in the general case.

Thanks!

[GF-Huang]

Any progress?

[laolarou726]

@Vect0rZ Any progress for this?