Read async input as IAsyncEnumerable<IAsyncEnumerable<string>>

[airbreather]

On CsvAsyncInputBase:

public IAsyncEnumerable<IAsyncEnumerable<string>> AsEnumerableFromUtf8()
{
    // ...
}

public virtual async IAsyncEnumerable<IAsyncEnumerable<string>> AsEnumerable(Encoding inputEncoding)
{
    // ...
}

[airbreather]

Base implementation probably also needs to spin up a thread like the sync version, because I chose not to implement async visitors in favor of a cleaner API.

Subclasses can almost certainly do a lot better, as in the sync version.