Add sample that uses Selector.AddSend

[cretz]

The docs currently say:

// The sending message to the channel is expected to be done by the callback function

But this is unclear.

[ioannist]

Indeed, the comments of the Selector interface are not very helpful and there are no examples that show how the different triggers differ and when they should be used.

https://github.com/temporalio/sdk-go/blob/master/internal/workflow.go