KAZOO-2615: play_welcome flag added to amqp discovery req

So I see the headers are here in the API, and I see conf_discovery checking them, but I don't see any apps (callflows?) setting them on the API request.

Also, since `Play-Welcome` is supposed to be a boolean, add a validator here:

`{<<"Play-Welcome">>, fun wh_util:is_boolean/1}`