Allow signaling of custom activities

[sfmskywalker]

This idea originates from a conversation I had with TedDBarr on Discord.

Currently, the /signals/trigger endpoint that is invoked by generated signal token URLs only triggers the SignalReceived activity,

But if we include the activity type from where one generates the token (e.g. WitingOnTaskCompletion in below sample custom activity) then the endpoint that the URL points to can read the activity type and trigger that one instead of SignalReceived.

Which is great, because it simplifies implementing custom "signal" activities such as your example tremendously!

public class WaitingOnTaskCompletion :  Activity
{
    protected override IActivityExecutionResult OnExecute(ActivityExecutionContext context)
    {
        var signalUrl = context.GenerateSignalUrl("Done");

        return Suspend();
    }
}

[promontis]

@sfmskywalker what is the state of this?

[sfmskywalker]

Still in the TODO state.