Improve introspection for SignalProducerWorkflow actions

[amorde]

This change improves the usability of #168 and also makes it easier to debug when inspecting arbitrary workflow actions.

Below is an example of the string interpolation result of a SignalProducerWorkflow action before and after this change:

func workflowDidReceiveAction<Action: WorkflowAction>(
    _ action: Action,
    workflow: Action.WorkflowType,
    session: WorkflowSession
) {
    print("\(action)")
}

// before
"AnyWorkflowAction<SignalProducerWorkflow<Output>>(_apply: (Function))"
// after
"Action(output: OrdersSDKImpl.UpcomingOrdersMutationWorker.Output.ordersDidMutate)"

The primary reason the output was not useful before was the usage of AnyWorkflowAction(sendingOutput:) which wraps the output value in a closure so it becomes inaccessible.

workflow-swift/Workflow/Sources/WorkflowAction.swift

Lines 67 to 71 in 8b7e97b

	public init(sendingOutput output: WorkflowType.Output) {
	self = AnyWorkflowAction { state in
	output
	}
	}

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation

[CLAassistant]

All committers have signed the CLA.

[jamieQ]

this seems like a good change to improve the utility of type-level introspection. do you think repeating the associated workflow's type in the action's type itself would be valuable? i.e. SignalProducerWorkflowAction or something. or putting that info into the string via 'custom string convertible' or whatever the appropriate conformance may be.

[amorde]

I did consider that yeah. I'll go ahead and make that change.

I also considered pulling a generic SendOutputAction out as a standalone type, but wasn't sure if that was worth it