Description

There are many existing APIs that returns a Progress instance, which can be used to track progress and some can be used to perform cancellation. This PR implements these features in the new URLSession helper.

The existing APIs return a Progress as function return value. We can't do the same, because the new helper is an async function: the Progress will be no use if it's used as a return value. I used a parent progress instance instead. The caller now needs to pass in a valid Progress instance that's created exclusively for the HTTP request, and it'll be completed upon the completion of the HTTP request.

Another approach I tried is using a inout Progress argument, so that we don't need to create a duplicated progress instance. I envisioned it can be used as below, but ended up getting a compiler error.

var progress = Progress(totalUnitCount: 100)
Task {
    // Compiler error. The `progress` reference can't be captured in this closure.
    let _ = await perform(request, &progress)
}
// Use the progress to track the HTTP request.
track(progress)

Testing Details

See the added unit tests.

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