Add Async Methods on CommandLineApplication that accept CancellationToken

[bloudraak]

Describe the bug
Consider the following code snippet:

public async Task<int> ExecuteAsync<TCommand>(string[] args, CancellationToken cancellationToken = default) where TCommand:class
{
    cancellationToken.ThrowIfCancellationRequested();
    
    var app = new CommandLineApplication<TCommand>();
    try
    {
        app.Conventions
            .UseDefaultConventions()
            .UseConstructorInjection(Services);

        return app.Execute(args);
        
    }
    catch (CommandParsingException e)
    {
        await app.Out.WriteLineAsync(e.Message);
        app.ShowHelp();
        return -1;
    }
}

The API doesn't expose ExecuteAsync(args, cancellationToken).

To Reproduce
There is no ExecuteAsync method that takes a cancellation token, which in turn complicates matters when TCommand implements an ExecuteAsync method, which uses a cancellation token.

Expected behavior
The expectation is that all methods on CommandLineApplication exposes async methods that accepts a cancellation token, so that a long running application can be cancelled.

Additional context
The need exists to signal to the instance of TCommand that is running asynchronously that processing should be cancelled for whatever reason, e.g. the user pressed CTRL+C.

[bloudraak]

In addition, async methods don't support passing in a cancellation token.

[natemcmaster]

Somewhat related: #111. This is new API I would consider adding in 3.0 which I plan to start work on later this year.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Please comment if you believe this should remain open, otherwise it will be closed in 7 days.

[natemcmaster]

Stale bot closed this, but I'm interested in adding this in 3.0. Reopening.