NIOThrowingAsyncSequenceProducer.AsyncIterator should support isolated next call

[fabianfett]

To make iteration of AsyncSequences inside actors more performant the function

func next(isolation actor: isolated (any Actor)?) async throws(any Error) -> Element

was added as an advanced implementation hook. NIOThrowingAsyncSequenceProducer.AsyncIterator would ideally implement it.

Implementing this should be fairly easy as isolation: isolated (any Actor)? = #isolation needs to be added to all internal next calls until the internal lock is used. This only works with a Swift 6 compiler though.