Extract the Clojure loading code to commons

It's currently inside the ring adapter while the same code should also be used for the background adapter.
https://github.com/pepijn/google-cloud-function-ring-adapter/blob/75abf37c817ad58cb88f965586fa0e970cf8acc4/src/java/nl/epij/gcp/gcf/RingHttpFunction.java#L44-L82