Offer CancellationToken support in IProgram (.NET 7)

[alexrp]

Depends on:

• Upgrade to net7.0 target framework (.NET 7) #103

Something like:

public interface IProgram
{
    public static virtual bool SupportsCancellation => true;

    public static abstract Task RunAsync(ProgramContext context);
}

If SupportsCancellation is true, ProgramHost will wire up a Signaled event handler to trigger a CancellationTokenSource whose CancellationToken gets passed as a property on ProgramContext.

(The reason for making cancellation support optional is that wiring up Signaled has side effects that may not be tolerable - see #64.)

[alexrp]

This can only happen in .NET 7 as the Roslyn shipped with .NET 6 does not support static virtual members.