make progress_token::operator() const so it can be captured by lambdas

[oldnewthing]

This is useful when you are wrapping another operation's progress, as is common with HTTP filters.

auto operation = m_inner.SendRequestAsync(request);
operation.Progress([progress_token = co_await get_progress_token()](auto&&, HttpProgress progress)
{
    progress.Retries += retries_performed;
    progress_token(progress);
});
co_await operation;

[kennykerr]

/azp run

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