Support returning slice/array of cloudevents from Receiver

[AlfredBerg]

Currently the following function signatures are supported when creating a Receiver:

sdk-go/v2/client/receiver.go

Lines 45 to 59 in 5aa0338

	// receiver creates a receiverFn wrapper class that is used by the client to
	// validate and invoke the provided function.
	// Valid fn signatures are:
	// * func()
	// * func() protocol.Result
	// * func(context.Context)
	// * func(context.Context) protocol.Result
	// * func(event.Event)
	// * func(event.Event) transport.Result
	// * func(context.Context, event.Event)
	// * func(context.Context, event.Event) protocol.Result
	// * func(event.Event) *event.Event
	// * func(event.Event) (*event.Event, protocol.Result)
	// * func(context.Context, event.Event) *event.Event
	// * func(context.Context, event.Event) (*event.Event, protocol.Result)

I currently have a use-case where I need to return multiple cloudevents from a function. I think I have found a way to do this by creating a client like cloudevents.NewClientHTTP(cloudevents.WithTarget(knativeTarget)) inside the function and then sending the events.

It would have been nice if I could simply have had a function definition like func(context.Context, event.Event) ([]*event.Event, protocol.Result) or func(context.Context, event.Event) ([]event.Event, protocol.Result) that returns a slice of events, that would then have handled the events.

If this is something that would make sense, I could have a go and see I if I can implement this