Expose configuration to define scheduling context on which an activity runs

[creyke]

Users requiring one or more of their activities to execute on a .NET thread pool thread, rather than via the Orleans task scheduler, should be able to express this per activity.

This is common if heavy compute is required, and/or if the user has no interest in interacting with the Orleans runtime from within their activity.

Example:

public class ArbitraryActivity : Activity<ArbitraryConfig>
{
    public void OnConfigure(ActivityConfig config)
    {
        config.ExecuteOnThreadPool = true;
    }
}

[creyke]

Current consumers have not expressed desire for this / dismissed it.