Amplify V2 will have an async API and encapsulate Operations as an internal implementation. `AmplifyTask` will define the public API for Tasks. Operations are given to `ChildTask` to provide async properties `value` and `inProcess`.
* adds AmplifyTask
* adds ChildTask
* revises AsyncChannel and AsyncThrowingChannel to use AsyncStream
* adds BufferingSequence to support AsyncStreams
* implements Task.sleep(seconds:)
* revises Combine support in Operations to work with AmplifyTask
* adds RequestIdentifier
* integrates RequestIdentifier with AmplifyTask
* adds AsyncPassthroughSubject to work with AmplifyTask
* adds AsyncExpectation to support testing
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