internal/runtimeconfig: initial prototype

Adding initial prototype of runtimeconfig package which provides ability
to be notified of runtime config changes from Cloud Runtime Configurator
service.

Change-Id: Id5765e15bc960fd001286ee14678c5ff8e2ec168
Reviewed-on: https://go-review.googlesource.com/69297
Reviewed-by: Ross Light <light@google.com>